Amit goes to his office by car at the speed of 80 km/hr and reaches 15...
Let the speed to reach on time be s.
Let the distance to office be d.
Time taken by car at 80 km/hr = x = d/s = d/80
Time taken by car at 60 km/hr = y = d/s = d/60
Time taken by car at s km/hr = z = d/s = d/s
Given: x = z – 15/60, y = z + 15/60
⇒ d/80 = d/s – 15/60 = d/s – 1/4 ⇒ d(1/80 – 1/s) = -1/4
⇒ d/60 = d/s + 15/60 = d/s + 1/4 ⇒ d(1/60 – 1/s) = 1/4
Dividing the two equations, we get
(1/80 – 1/s)/(1/60 – 1/s) = -1
⇒ 1/80 – 1/s = 1/s – 1/60
⇒ 2/s = 1/80 + 1/60 = 1/20(1/4 + 1/3) = (1/20)(7/12)
⇒ s = 40 × 12/7 = 480/7 km/h