Thursday, October 21, 2010

Eleven Plus Probability.

Some parents need to be very proud of themselves. You may well have played the game, even with your eleven plus child, where you ask your child to guess which hand you are holding something desirable.

The first time your child may choose the left hand.

For the next three times he or she may choose the same hand as the previous time.

The same hand gives you probability `s’.

The different hand gives probability `d’.

You then suggest that s + d = 1.

Your task then is to work out the probability that he or she will choose `s’ on the last occasion.

Parents always know best and will be able explain this lucidly to their eleven plus child – who will not say: “What?”

The mums and dads who did `A’ Level statistics at school or university will no doubt explain that (s + d) to the power of 3 and (s – d) to the power of 3 can be written as:

A half of (1 + (s – d) to the power of 3).

Other mums and dads might want to give a simpler answer. Both sets could be right!