Correct option is (c) 25 km/hr
Let the distance between the two town by road be x km
∴ Distance by train is (x + 20)km
Average speed of car be y km/h
∴ Average speed of train is (y + 15)km/h.
Given,
\(y = \frac x4\) .....(i)
\(y + 15 = \frac{x + 20}3\) ......(ii)
⇒ 3y + 45 = x + 20
⇒ 3y − x = 20 − 45
⇒ \(3(\frac x4) \) - x = -25 [Putting the value of y from equation (i)]
⇒ \(\frac {3x}4\) - x = -25
⇒ \(\frac{3x - 4x }4 = -25\)
⇒ -x = -100
⇒ x = 100
Putting the value in equation (i), we get,
\(y = \frac x4\)
⇒ \(y = \frac{100}4\)
⇒ y = 25
∴ Average speed of the car is 25 km/h.
