Lets call the distance between a and the school 'x'
so the total distance traveled = 300x + 700(5-x)
we want to minimize the function y = 300x + 700(5-x)
which simplifies to
y = 300x + 3500 - 700x
y = -400x + 3500
To minimize the function we need to maximize x, x=5 is the highest number that makes sense.
therefore the school should be in town B.