ARITHMATIC RUSES

INCREDIBLE BIRTHDAYS

If there are 23 people in a room the chance that any two of them will

have the same birthday is actually higher than 50%. Now you can

use statistics to impress your friends!

PALINDROME NUMBERS

By reversing a number and adding it back to itself over and over you

can make almost any number a palindrome. Here is an example:

525600 + 6525 = 532125

532125 + 521235 = 1053360

1053360 + 633501 = 1686861

LYCHREL NUMBERS

There are some numbers you cannot do the last trick with though.

At least no computer has been able to find a palindrome yet.

The lowest known lychrel number is 196.

ADDING 5

Although it seems counterintuitive it has been shown that people can

add 5 to any number greater than 5 if they subtract 5 and then add 10.

For example, 8 + 5 would be 8 – 5 = 3 and 3 + 10 = 13.

MULTIPLYING BY 11

To multiply a 2 digit number by 11 just take the sum of its digits.

If it is a single digit number just right it between the digits.

If it is greater than 2 digits, carry the 1! Here are some examples:

34 x 11 = 374

47 x 11 = 517

MULTIPLYING BY 9

When multiplying by 9 simply multiply by 10 and then subtract the

other number. For example:

23 x 9 = 230 – 23 = 207

RULE OF 72

In financial mathematics this is a quick way to figure out how long it

will take an investment to double given a fixed annual rate of return.

For example, $1 invested at 10% would take 7.2 (72/10)

years to double and turn into $2.

TURN REPEATING DECIMALS INTO FRACTIONS

This can be frustrating even with a calculator but there is a trick!

Let’s take 0.63636363… First, find the repeating part of the

decimal (63). Divide the repeating part by another number that has

the same number of place but consists of nines (99).

So 0.63636363… equals 63/99