Maths: Magic Maths

[HwFactory]

The Magic Recurring Number

The Trick
Take a pen and a piece of paper and write down the following 8 digitnumber.

1 2 3 4 5 6 7 9

Ask a friend to circle one of the digits.

For example, if they circle number 4 then ask your friend to multiply the 8 digit number by 36. The result will be:

1 2 3 4 5 6 7 9
x 36
4 4 4 4 4 4 4 4 4

Lets try it again, lets imagine that they choose the number 9, ask them to multiply the 8 digit number by 81. The result will be:

1 2 3 4 5 6 7 9
x 81
9 9 9 9 9 9 9 9 9

The Secret

When your friend circles a number multiply it by 9. So if they choose 2, ask them to mupltiply the 8 digit number by 18. If they choose 5 ask them to multiply the 8 digit number by 45. Whichever number they choose, if you follow this rule then they will get a magic recurring number as the result.



Addition at the Speed of Light

The Trick

Get a friend to write down any 2 numbers between 1 and 19 (including 1 and 19). When they have 2 numbers, tell them to add the 2 numbers together to get a third, then add the second and third to get a fourth, the third and fourth to get a fifth, and so on until there are 10 numbers. Now you can amaze them with the speed of your addition.

How?

To quickly get the sum of the 10 numbers, multiply the seventh number by 11.

As an example, my friend wrote 11 and 13, so the sequence of numbers goes:

11, 13, 24, 37, 61, 98, 159, 257, 416, 673

The sum of this sequence is 1749. The result of 159 (the seventh number) multiplied by 11 is 1749.

If you are having trouble multiplying by 11 then remember this quick solution. Multiply the number by 10, then add the number.

So lets pick a random number, say 389. 389 x 11 = (389 x 10) + 389 = 3890 + 389 = 4279