Question

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

Answer 1

Speed =distance /time
Average speed=total distance /total time
=54km/hr =15m/s
For 20s,
V1 =( 15*20)m
=300m
36km/hr=10m/s
For 30s,
V2=(10*30)
=300m/s
18km/hr=5m/s
For 10s,
V3=(10*5)
=50m/s
Total distance =300m +300m+50m
=650m
Total time taken =20s+30s+10s
=60s
Average speed =650/60m/s
=65/6m/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!