To find probability, you write a fraction with the numerator as the subject you are trying to find the probability of and with a denominator as the total number of subjects. The total number of subjects is 35 (or 15 + 20). The subject we are finding the probability of is men, which there are 15 of. Therefore the fraction would look like this:
15/35
which we can simplify to
3/7
The probability that the next person to enter the store will be a man is 3/7