A If there is sufficient stock to last 60 students 15 days, then each student has enough for 15 days and the stock is 60*15=900 student-days. If there is only one student the same supply will last that student 900 days.
B If 900 student-days is the stock, then 900/36=25 is the number of students that can be provided for so that the food lasts for just 36 days.
C An extra 15 students makes the total 75 students. So the number of days the stock will last is 900/75=12 days.