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.

Options

A. 26.7 Km/h
B. 2.67 Km/h
C. 48.7 Km/h
D. 267 Km/h

Answer

A

Explanation

Here , distance , `s_(1) = 30 km` , speed `v_(1) = 40 km //h`
distance , `s_(2) = 30 km` , speed , `v_(2) = 20 km//h`
If ` t_(1)` and `t_(2)` the respective values of the time taken by car , then
`t_(1) = ( s_(1))/( v_(1)) = ( 30)/( 40) = 0.75 h` and `t_(2) = ( s_(2))/( v_(2)) = (30)/( 20) = 1.5 h`
As average speed `= ("total distance travelled")/( "total time taken")` ,
` v_( av) = ( s_(1) + s_(2))/(t_(1) + t_(2)) = ( 30 + 30)/( 0.75 + 1.5) = ( 60)/( 2.25) = 26.7 km//h`.