How would you do this and what subject?

Aclothing store has just advertised at a bargain price, that a customer tries on a t-shirt is 0.40. a new range of t-shirts. the probability if a customer tries a t-shirt, the probability that they will buy it is 0.70 while if a customer does not try on a t-shirt, the probability that they will buy it is 0.15. a customer has just bought a t-shirt. find the probability that the customer did not try on the t- shirt.