The way I see it, 325 students went to the trip and 5 had to go in cars so 325 - 5 = 320 then you devide it by 7 because that's the total number of buses which you should get 320/7 Which would equal to 45 students on each bus..
325 - 5 = 320/7 = 45
Hope this helps!
(Btw I rounded to 45 because the whole number was like 45.71428571428571)