How do you do this question?

The answer is D) 12

There are two trips, but the distances are the same.

Trip #1: uphill, at 9 mph

Trip #2: downhill at 18 mph

Total round trip is 30 minutes.

Since he's going half the speed uphill, Trip #1 takes twice as long as Trip #2.

So if the time for Trip #2 is x, then the time for Trip #1 is 2x.

     x + 2x = 30, so x=10

Trip #2 takes 10 minutes.

Trip #1 takes 20 minutes.

Trip #1: 20 minutes at 9 mph = 3 miles

Trip #2: 10 minutes at 18 mph = 3 miles

This means his school is 3 miles away.

If you want an equation to solve,

Let d = distance to school.  We use d = r·t formula to set up that t = d/r

let t1 = time going to school and t2 = times going home

t1 + t2 = 30 and t1 = d/9 and t2 = d/18.

    d/9 + d/18 = 0.5    (it's 0.5, because we need the time in hours)

      mutiply both sides b 18

    2d + d = 9

     3d = 9

       d = 3