Correct Answer - Option 1 : 8 km/h
Given:
Distance travelled = 120 km
Increase in speed = 4 km/h
Difference in time taken = 5 hrs
Formula used:
Time taken = Distance/speed
Calculation:
Let the initial speed = x km/h
∵ Initial time taken - New time taken = 5 hrs
⇒ 120/x - 120/(x + 4) = 5
⇒ 120x + 480 - 120x = 5x(x + 4)
⇒ 480 = 5x2 + 20x
⇒ x2 + 4x - 96 = 0
⇒ x2 + 12x - 8x - 96 = 0
⇒ x(x + 12) - 8(x + 12) = 0
⇒ (x - 8)(x + 12) = 0
∴ x - 8 = 0
⇒ x = 8 km/h
x + 12 = 0
⇒ x = (-12) km/h Not possible
∴ Actual speed = 8 km/h