Two Girls

There are 10 girls in a mixed class. If two pupils are selected, the probability that they are both girls is 0.15. How many boys are in the class?
Exploring and noticing Working systematically Conjecturing and generalising Visualising and representing Reasoning, convincing and proving
Being curious Being resourceful Being resilient Being collaborative

Problem



There are $10$ girls in a mixed class.

If two pupils from the class are selected at random, then the probability that both are girls is $0.15$

How many boys are in the class?

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.