'Lo and behold, you have predicted it in the envelope!'

By Andrew Chapman.

The theme of this year's Christian Aid Week is 'You add, we multiply'. Andrew Chapman offers a few mathematical tricks to bamboozle your friends...

Trick 1

1. Out of sight, write down the number 18 on a piece of paper and seal it in an envelope
2. Ask your victim to pick 3 numbers from 1 to 9
3. Get them to write them down next to each other from largest to smallest to make a 3-digit number
4. Now reverse that number and write it underneath
5. Get them to subtract the lower number from the upper one
6. Now tell them to add the digits of the number that results together: lo and behold, you have predicted it in the envelope!

Trick 2

1. Assemble a group of people to impress with your mind reading
2. Give each of the people a different consecutive number
3. Get a piece of string and tell them to tie it on someone's finger while your back is turned
4. Get them to answer these questions in order and give you the answer after (g):
(a) multiply the number of the person with the string by 2
(b) add 3
(c) multiply the result by 5
(d) if the string is on their left hand, add 9
(e) multiply by 10
(f) add the number of the finger the string is on (thumb = 1, etc)
(g) add 2
5. Mentally subtract 222 from the answer. The first digit (or first two if there are 4 digits in the answer) gives the number if the player, the middle digit is 1 for the right hand (left hand otherwise) and the last digit tells you which finger the string is on!

Trick 3

1. Ask your victim to shuffle a standard deck of playing cards
2. Get them to pick a 'secret number' between 1 and 10
3. Ask them to deal out the cards one by one, face up, into a pile on the table, counting them silently according to this rule: when they have dealt the same number of cards as their secret number, the value of the latest card becomes the new 'key number', and they then count that many cards (NB: all picture cards count as 5). This gives a new key number, and so on until all 52 cards have been dealt. The value of the last key card becomes the 'magic number' that you will divine.

The trick is to pick your own secret number, ideally a low one, and do exactly the same as your victim, cycling through different key numbers. Eventually your sequence and theirs will coincide: there is around a 5 in 6 chance that you will arrive at exactly the same magic number, even though you never knew what the victim's original secret number was! (Watch out for the card as the victim nears the end of the pack and you will be able to tell them the suit as well as the number.)

• Christian Aid Week 2006