NCERT Question

Question

A car moves with a speed of 40 km/h for 15 minutes and then with a speed of 60 km/h for the next 15 minutes. The total distance covered by the car is 

(a) 100 km

(b) 25 km

(c) 15 km

(d) 10 km

Chapter: Motion and Time

Class: 7

What will be the answer for this with it's full explanation.

Answer 1

Correct option is (b) 25 km

Given,

Let the speed be 'S' and time be 't' respectively.

S₁ = 40 km/hr, S₂ = 60 km/hr, t₁ = t₂ = 15 min ⇒ 15/60 = 0.25 hr.

Since the journey has variable speed, therefore we have to calculate the distance separately for each case.

The total distance covered is D.

D = D₁ + D₂

D = S₁t₁ + S₂t₂ = (S₁ + S₂)t  [∵ t₁ = t₂ = t]

D = (40 + 60) x 0.25

D = 100 x 0.25 ⇒ 25 km.

∴ the total distance covered is 25 km.