A motorist covers a distance of 39km in 45min by moving at a speed of x kmph for the first 15min, then moving at double the speed for the next 20 min, and then again moving at his original speed for the rest of the journey. Find x.

Question

( A ) 58 km/h
( B ) 48 km/h
( C ) 36 km/h
( D ) 60 km/h