A motorist travels between two towns. It usually takes 6 hours when travelling at a certain speed. He finds that if he reduces his speed by 15 km/hr his journey will take 1 hour more. Find his usual speed.

His usual speed is 105 km/hr.

Step-by-step explanation:

Let his usually speed be s km/hr and the distance between the towns be d km. So we have the equations:

s = d/6

s - 15 = d / (6 + 1)

from the first equation:

d = 6s and from the second:

d = 7 (s - 15)

Eliminating d we get:

6s = 7 (s - 15)

7s - 105 = 6s

s = 105.