Question

John runs a computer software store. Yesterday he counted 127 people who walked by his store, 58 of whom came into the store. Of the 58, only 25 bought something in the store. Estimate the probability that a person who walks by the store will come in and buy something (put your answer as a fraction).

Answer 1

So probabilities are defined as "what we want to happen" divided by "all of the options".  127 is all the people who walked by.  25 is all of the people who did what we want.  So, 25/127 = .1969 or 19.69%.

Strangely, one could ponder this question in another way.  One could first get the probability that they walk in (58/127) and then multiply it by the probability that they buy something (25/58).  This turns out to give you the same answer.