If on average bob can make a sale to every 3rd person that comes into his store and he wanted to make approximate 15 sales?

(I apologise since i didn't understand the question very well, but here's what i think it means)

Answer = 45 people.

If Bob can only make a sale to every third person, then in order to make 15 sales he needs to multiply 15×3 in order to get the number 45. Which means, that in order for Bob to make 15 sales he needs to talk to 45 customers.

- Hope it helps!