Mole day is celebrated every year on October 23 from 6:02 am to 6:02 pm. This holiday honors chemistry’s own measurement unit corresponding to Avogadro’s Number: 6.02 x 1023. You can spend the day learning more about the mole, play a few rounds of “Whack a Mole” or have some guacamole. What better way to spend Mole Day than sharing a few mole puns with our friend Chemistry Mole.
Math is like magic, except better! Here is a collection of cool math tricks you can learn to amaze your friends and learn more about how numbers work.
The 11 Rule
Everyone knows the “10 Rule” where you multiply by 10 simply by adding a 0 to the end of a number, but you might not know the 11 rule. This rule works for any two digit number to multiply it by 11:
For an example, let’s use the number 62
- Separate the two digits in your mind (6 __ 2).
- Add together the two digits of the numbers. (6 + 2 = 8).
- Place this number in the space or hole between the two digits (6 8 2).
- That’s it! 11 x 62 = 682
The only tricky part to remember is that if adding the two digits results in a number greater than 9, then you put the “ones” digit in the space and carry the “tens” digit. For example:
11 x 57 … 5 __ 7 … 5 + 7 = 12
so you put the 2 in the space and add the 1 to the 5, giving you the number 627
11 x 57 = 627
Single Digit Numbers Math Trick
- Think of 2 single digit numbers.
- Take either of the numbers and double it.
- Add 5 to the result.
- Add the second number to your answer.
- Subtract 25 from the answer.
- You’ll get the 2 single digit numbers in the answer.
Using Shoe Size To Tell Your Age – Algebra Trick
There are many math tricks that ask you to supply a number to get a “hidden” number that you actually supply in a different form during the trick. The first number isn’t too important, since it gets removed during the trick, so you can change the wording of this math trick.
- Use your shoe size to tell your age. Take your shoe size (whole number, so round up if it’s a half size).
- Multiply it by 5.
- Add 50.
- Multiply it by 20.
- Add 1016. (if you’re doing the trick in the year 2016… if it’s 2017 use 1017, in 2018 use 1018, etc.)
- Subtract the year you were born.
- The first digit is your shoe size and the last 2 digits are your age.
As you might guess, this trick is meant to reveal a number that is less than 100. The trick uses algebra to solve for the answer. Let’s do the trick again using s for shoe size and b for birth year:
Multiply s x 5: 5s
Add 50: 5s + 50
Multiply by 20: 20(5s + 50) = 100s + 1000
Add 1016 (depending on current year): 100s + 1000 + 1016 = 100s + 2016
Subtract birth year: 100s + 2016 – b
Why does it work (and why does it sometimes fail)? No matter what your shoes size is, it will be the first two digits of the answer. If your shoe size is 9, the 100s is 900. If you use a European size chart and wear a size 36, then 100s = 3600.
The age part takes the current year minus your birth year. The trick does not take into account your birth month, so if your birthday this year has not arrived, the answer will be a year off!
1, 2, 4, 5, 7, 8 Math Trick
- Choose a number between 1 and 6.
- Multiply the number by 9.
- Multiply the result by 111.
- Multiply the answer by 1001.
- Divide the number by 7.
- The answer will contains all the numbers 1, 2, 4, 5, 7, 8.
The 1089 Math Trick
- Think of a 3 digit number.
- Arrange the digits in descending order.
- Reverse the order and subtract it from the number in step 2.
- Reverse the order of the answer.
- Add it to the result from step 3. You get 1089!
- For example, let’s say I chose 423.
- Arrange in descending order: 432
- Reverse the order and subtract it from the previous number: 234… 432 – 234 = 198
- Reverse the order: 891
- Add the numbers together: 198 + 891 = 1089
The Answer Is 5
- Think of a number.
- Double it.
- Add 10.
- Divide it by 2.
- Subtract your original number.
- Your answer is 5!
Three Digits the Same Trick
- Think of a 3 digit number where all the digits are the same (e.g., 333, 777).
- Add up the digits.
- Divide your 3-digit number by the added value.
- Your answer is 37.
How To Solve Cheryl’s Birthday Math Riddle
“Cheryl’s Birthday” is more of a math riddle or logic puzzle that is solved using the process of elimination or deductive reasoning. Albert can’t know the birthday because he only has the month and all months have multiple dates, but he has enough information to know Bernard does not have the date. If Cheryl had told Bernard 19 or 18, then he would know the whole birthday because there is only one month with each number. This rules out May 19 and June 18.
Since Albert knows Bernard doesn’t know, Albert must have been told July or August, as this rules out any possibility of Bernard being told 18 or 19. This excludes any dates in May or June.
When Bernard says he did not know the answer, but now he does, this means Bernard has the one remaining unique number in the list. If Bernard had a 14, he wouldn’t know whether it was in July or August. If he had a 15 or a 17, he wouldn’t know which date in August was correct. Thus Cheryl’s birthday must be July 16!
If you want to get really tricky, you can rephrase the riddle to say Bernard starts out saying he doesn’t know when the birthday is, with Albert replying he doesn’t know either. If Bernard then says he didn’t know, but now he does, and Albert replies he now knows too, then the answer is August 17. Can you see why?
Do you have other cool math tricks to add? Post a reply and share them!
Engineering is a line of work where mistakes can be costly on many levels. You would think engineers are a serious bunch of people, but there are an awful lot of engineer jokes out there. Here is a list of some of my favorite engineer jokes. Feel free to share yours in the comments if you have a good one.
Definition of an engineer: Someone who solves a problem you didn’t know you had, in a way you don’t understand.
Engineering is all about laziness in the name of better life.
Arguing with an engineer is a lot like wrestling in the mud with a pig. After a few hours, you realize that he enjoys it.
Q: Why do computer engineers get Halloween and Christmas mixed up?
A: Because 31OCT = 25DEC.
An engineer is a fellow that takes a measurement with a micrometer, marks it with a crayon,
and cuts it with an axe.
During the French Revolution, three men were condemned to the guillotine. One was a preacher, one was a doctor, and the third was an engineer. When the preacher approached the deadly machine, he requested to be beheaded while lying on his back so that he could die while looking into Heaven. The doctor and the engineer thought that to be a good idea and requested the same. As the blade plunged down the track toward the preacher, it suddenly stopped just short of the man’s neck. The executioner declared it an Act of God and let the man go free. The same thing happened to the doctor. As the engineer laid his head back in place he suddenly said, “Oh wait! I see the problem!”
The optimist sees the glass as half full. The pessimist sees the glass as half empty. The engineer sees the glass as twice as big as it needs to be.
Q: What is the difference between Mechanical Engineers, Civil Engineers and Chemical Engineers?
A: Mechanical Engineers build weapons; Civil Engineers build targets. Chemical Engineers are engineers that build targets that explode really well.
A wife asks her engineer husband, “Could you please go shopping for me and buy one carton of milk, and if they have eggs, get 6!”
A short time later the husband comes back with six cartons of milk.
The wife asks him, “Why in the world would did you buy 6 cartons of milk?”
He replied, “They had eggs.”
Q: What’s the difference between a chemical engineer and a chemist?
A: A chemical engineer does for profit what a chemist does for fun.
A programmer and an engineer were sitting next to each other on an airplane. The programmer leans over to the engineer and asks if he wants to play a fun game. The engineer just wants to be left alone, so he politely declines, turns away, and tries to sleep.
The programmer continues to pester the engineer. “C’mon, it’s a real easy game. I ask a question and if you don’t know the answer you pay me $5. Then you ask me a question and if I don’t know the answer I’ll pay you $5.” Again, the engineer declines and tries to sleep.
The programmer really wants to play the game and says, “OK, if you don’t know the answer, you pay me $5, and if I don’t know the answer, I’ll pay you $50!”
The engineer grins and agrees to play. The programmer asks the first question, “What is the distance from the earth to the moon?”
The engineer doesn’t say a word and just hands the programmer $5.
The engineer asks the programmer, “What goes up a hill with three legs and comes down on four?” The programmer looks thoughtful, takes out his laptop and starts to surf the net for the answer. After an hour he wakes the engineer to hand him $50. The engineer takes the money, turns away, and tries to go back to sleep.
The programmer asks, “Well? What’s the answer to the question?”
Without a word, the engineer reaches into his wallet, hands $5 to the programmer.
A group of physicists and a group of engineers were travelling by train. Each physicist had their own train ticket while the engineer group had only ONE ticket for all of them. The physicists started laughing, figuring the engineers were going to get caught and thrown off the train.
One engineer spots the conductor coming and they all duck into the bathroom. The physicists were puzzled.
The conductor came aboard, said, “Tickets, please,” and got tickets from all the physicists. He then went to the bathroom and knocked on the door and said, “Ticket, please.” The engineers stuck the ticket under the door. The conductor took it and moved on. A few minutes later, the engineers came out of the bathroom. The physicists felt really stupid.
On the return trip, the physicists decide they should try the engineer’s scam. They bought one ticket for the whole group. They met up with the engineers in the same car.
The physicists all smile and wave their lone ticket. This time, the engineers have no ticket. One engineer nods towards the door between cars and says, “Conductor coming!”. The engineers sat back as all the physicists locked themselves into the bathroom.
After a moment, one engineer knocks on the bathroom door and says, “Ticket, please.”
A physicist, a mathematician and an engineer were each asked to establish the volume of a red rubber ball.
The physicist immersed the ball in a beaker full of water and measured the volume of the displaced fluid.
The mathematician measured the diameter and calculated a triple integral.
The engineer looked it up in his Red Rubber Ball Volume Table.
Most people believe “If it ain’t broke, don’t fix it!”
Engineers believe “If it ain’t broke, add more features!”
Two engineering students were walking across a university campus when one said, “Where did you get such a great bike?”
The second engineer replied, “Well, I was walking along yesterday, minding my own business, when a beautiful woman rode up on this bike, threw it to the ground, took off all her clothes and said, “Take what you want.”
The first engineer nodded approvingly and said, “Good choice; the clothes probably wouldn’t have fit you anyway.”