## Sunday, January 13, 2013

### Eleven Plus Probability

Early accounts of Australia told stories of men gambling with `Two Up’.  Two coins were tossed into the air and there were bets on the results. Some of the bets were small – but others were life changing.

If there were two heads the thrower won. With two tails the thrower lost. If there was a head and a tail then the throw was made again.

No self -respecting eleven plus parents would ever want to gamble with their child – but it could be an interesting way of a child learning about the laws of chance and probability. (Is playing for matches gambling?)

Questions on probability could, possibly, come up in the eleven plus. Consider a question along these lines:

You look at the boys and girls in your class of 25. Some are wearing glasses. You are asked to make an analysis.

 Boys Girls Glasses 4 5 No Glasses 5 11

The person sitting beside you is a girl. She wears glasses.
Nine people wear glasses so the probability of her wearing glasses is 9 out 25.
Only 4 boys wear glasses. This suggests 4 out of 9.

What is the probability of a girl wearing glasses?

What is the probability that the girl sitting beside you will be wearing glasses?

You could also explore probability using a table.

Probability is measured on a scale – where zero represents impossibility and one represents certainty.

 Passing the eleven plus while asleep Being either a boy or a girl 0 0.25 0.5 0.25 1 One coin landing either heads or tails