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 
54 km/hr=15 m/s-1
36 km/hr=10 m/s-2
18 km/hr=5 m/s-3(We multiply all 3 of them by 5/18 to convert into m/s)

For 1 
15 m/s=d1/20
300 m=d1 
For 2 
10 m/s=d2/30
d2=300 m
For 3 
5 m/s=d3/10
50 m=d3
Since total distance/total time =average speed
d1+d2+d3/t1+t2+t3=Average speed
=>300+300+50 m/60 s
=650/60 m/s or
10  5/6 m/s is your average speed

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!