Cool Math tricks

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

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.

1. Use your shoe size to tell your age. Take your shoe size (whole number, so round up if it’s a half size).

2. Multiply it by 5.

3. Add 50.

4. Multiply it by 20.

5. Add 1016. (if you’re doing the trick in the year 2016… if it’s 2017 use 1017, in 2018 use 1018, etc.)

6. Subtract the year you were born.

7. 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!

Tip to Remember Pi

Pi is an irrational number, so you can’t learn all of its digits. However, you can learn the first few for calculations. Just remember the sentence:

“How I wish I could calculate pi.”

The number of letters in each word is the digit, giving you 3.141592.

The Answer Is 5

1. Think of a number.

2. Double it.

3. Add 10.

4. Divide it by 2.

5. Subtract your original number.

6. Your answer is 5!

Three Digits the Same Trick

1. Think of a 3 digit number where all the digits are the same (e.g., 333, 777).

2. Add up the digits.

3. Divide your 3-digit number by the added value.

4. Your answer is 37.