Question

a motorist covers 39 km in 45 mins of his journey by moving at the speed of x km/ph for first 15 mins. then moving at double the original speed for next 20 mins and then again moving at his original speed x km/ph for the rest of the journey. now, x=?

Answer 1

#For the First 15 minutes (= 1/4 hours) of journey :
speed = x km/h
=> distance travelled in this part of journey = speed × time
= x× 1/4 = x/4 Km
_________________

#For Next 20 minutes (= 1/3 hours) of journey :
speed = 2x Km/h
(as speed becomes double;given )
=> distance travelled in this part of journey : (2x)×(1/3) = 2x/3 Km
_________________

#For next remaining time [ 10 minutes (=1/6 hours) ] of journey :
speed = x Km/h
( moves with original speed ;given)
=> distance travelled in this part of journey = x×1/6 = x/6 Km
_________________

The total distance the motorist cover = 39Km .
=>
(x/4)+(2x/3)+(x/6) = 39 Km
=> 13x = 39×12
=> x = 36 Km/h