A sign on the gas pumps of a chain of gasoline stations encourages customers to have their oil checked, claiming that one out of four cards need to have oil added. If this is true, what is the probability of the following events?

Question

a. One out of the next four cars needs oil.
b. Two out of the next eight cars need oil.
c. Three out of the next 12 cars need oil.
I can not figure out how to set this up mathematically.