If A travels to his school from his house at the speed of 3 km/h, then he reaches the school 7 minutes late. If he travels at the speed of 4 km/h, he reaches the school 8 minutes earlier than school time. The distance of his school from his house is:

Question: If A travels to his school from his house at the speed of 3 km/h, then he reaches the school 7 minutes late. If he travels at the speed of 4 km/h, he reaches the school 8 minutes earlier than school time. The distance of his school from his house is:

Solution:
Let the distance between school and home be x km.
The difference of time when A goes school to school with these two different speeds is = 8 - (- 7) = 15 min
=15/60 hour

Now
(x/3) - (x/4) = 15/60
(4x - 3x)/12 = 1/4
x/12 = 1/4
x = 12/4
x = 3