Question

A car travels at 54 km/h for first 20s, 36km/h for next 30s and finally 18 km/h for next 10s. Find its average speed.

Answer 1

54 km/hr = 54 x 5/18 = 15 m/s
36 km/hr = 36 x 5/18 = 10 m/s
18 km/hr = 18 x 5/18 = 5 m/s

1st Case ---
Speed = 15m/s
Time = 20 s
Distance = Speed x Time = 15 x 20 = 300 m

2nd Case ---
Speed = 10 m/s
Time = 30 sec
Distance = 10 x 30 = 300 m

3rd Case ---
Speed = 5 m/s
Time = 10s
Distance = 5 x 10 = 50 m

Now Total Distance = 300 + 300 + 50 = 650 m
Total time = 20 + 30 + 10 = 60 s

Average Speed = 650/60 = 65/6 m/s

Answer 2

Hey there!

Formula used:

• Distance = speed * time

• Speed ( in 1 km/h) = 5 /18 (in 1 m/s)

Distance covered when car travels at 54 km/h for first 20 s:

Speed = 54 km / h

           = 54 *( 5/18) m/s

           = 15 m/s

Distance (d₁) = speed * time

                    = 15 * 20 = 300 m

Distance covered when car travels at 36 km/h for  30 s:

S = 36 km/h = 36 * (5/18) m / s  = 10 m/s

d₂ = v * t = 10 * 30 = 300 m

Distance covered when car travels at 18 km/h for  10 s:

S = 18 km/h = 18*(5/18) m/s = 5 m/s

d₃ = v * t = 5 * 10 = 50 m

Average speed = Total distance(d₁ + d₂ + d₃) / Total time

                           = (300 + 300 + 50) m / (20 + 30 + 10) s

                           = (650 m) / 60 s

                           = 10.83 m/s  (←Answer)

Good day!