Question

.5 A car travels 30 km at a uniform speed of 40 km/h and the next 30 km at a uniform speed of 20 km/h
Find its average speed

with solution​

Answer 1

We know that,

Speed=Distance / time

.°. Time =dist / speed

Let's see the first case,

s = distance = 30 km

v = speed = 40 km/hr

t = time = 30 / 40

=3/4 hrs

Let's see the second case,

s = 30 km

v = 20 km/hr

t = 30 / 20

t = 3/2 hrs

Avg. Speed =Total distance/ Total time

=(30 + 30) / (3/4) + (3/2)

=60 / ( 3 + 6 ) /4

=60 / 9 × 4

=20 / 3 × 4

=80 / 3

=26.67 ( Approx. )

Hence,

Average speed of the car is 26.67 km/hrs ( Approx. )

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Answer 2

Average speed = Total distance covered / Total time taken

Time taken to travel the first 40 km = 1 h  

Time taken to travel the next 30 km = 30km /(20 km/h) = 1.5 h

Total distance travelled = 40 + 30 = 70 km

Total time taken = 1 + 1.5 = 2.5 h

Average speed = 70 km/ 2.5 h

                           = 28 km/h