For the purpose of showing the solution to this item, we let the basis in which we are to determine the number of trains that passed through be 1 day. That being said, we convert 1 day to number of minutes by using the correct conversion factor and dimensional analysis as below.
(1 day)(24 hours/1 day)(60 minutes/1 hour) = 1440 minutes
Then, divide the calculated number of minutes by 20.
number of trains = 1440 minutes x (1 train / 20 minutes)
number of trains = 72 trains
Answer: 72 trains