Hi Chan,
I am assuming you are referring to the solution for question 2?
In the answer the number of passengers in April, May and June is calculated as ¼ of the yearly number of passengers.
You could also multiply the daily number of passengers by 30 (to get to the monthly number) and than multiply by 3 (to get to the number for April, May and June).
Hope this helps, otherwise it would be helpful to be more specific where you are exactly struggling to get the answer