Two trains leave the same station at the same time. The first train travels north at 60 mph. The second train travels west at 45 mph. Find how fast the distance between the trains is changing two hours after the trains have left.

15 mph

Explanation:

Velocity = Distance/time

Distance, D = velocity * time

After 2 hours, the first train would have covered a distance of:

D1 = 60 * 2 = 120 miles

The second train would have covered a distance of:

D2 = 45 * 2 = 90

The difference in the distance covered by the two trains after 2 hours is:

ΔD = D1 - D2

ΔD = 120 - 90

ΔD = 30 miles

The rate at which the distance is changing, Rate = ΔD/Δt

Rate = 30/2

Rate = 15 mph