ARITHMATIC RUSES
Added on: 27th Oct 2015
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
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