The answer would be something like:
(173+124)/3=99
Given that there are 173 third graders and 124 fourth graders, what we would need to do is find the total which would be equal to 297 students.
Then because each seat on the bus fits 3 students, we divide the total by 3 to get the estimated number of seats needed, which gives 99.
So, 173 + 124 = 297 students
297/3 = 99 seats