Question

a car moves at a constant speed of 40 km/hr for 1 km and 20 km/hr for the next 1 km . What is the average speed?

Answer 1

Answer:

FIRST HALF

distance travelled : 1km

speed : 40km/hr

time taken = displacement/speed

                  = 1/40 hr

SECOND HALF

distance travelled : 1km

speed : 20km/hr

time taken = 1/20 hr

AVERAGE SPEED = TOTAL DISPLACEMENT / TOTAL TIME

                               =   1 + 1 / (1/40 + 1/20)

                               =   2 / (3/40)

                               =    2*40/3

                               =    80/3

                               =    26.6 km/hr

Answer 2

Given :-

Speed in for the first travel = 40 km/h

Distance travelled for the first travel = 1 km

Speed for the second travel = 20 km/h

Distance travelled for the second travel = 1 km

To Find :-

The average speed of the car.

Solution :-

We know that,

• s = Speed
• d = Distance
• t = Time

CASE I :-

Speed (s) = 40 km/h

Distance (d) = 1 km

Time (t) = d/s = 1/40 km/h

CASE II :-

Speed (s) = 20 km/h

Distance (d) = 1 km

Time (t) = d/s = 1/20 km/h

Using the formula,

$\underline{\boxed{\sf Average \ speed= \dfrac{Total \ distance}{Total \ time} }}$

Given that,

Distance (d) = 1 + 1 km

Time (t) = 1/40 + 1/20

Substituting their values,

⇒ (1+1) / (1/40+1/20)

⇒ (2 × 800) / 100

⇒ 1600 / 100

⇒ 16 km/h

Therefore, the average speed of the car is 16 km/h.