plz tell

let the original speed be x km/hr

speed =dist/time

time=dist/speed

 so to cover a dist of 90 kms at the speed x km/hr it would take

90/x   hrs

if the spped is increased by 15 km/hr then the new speed will be (x+15) km/hr

the time taken to cover  90 km with this speed, it would take

90/(x+15) hrs

the time diff in both cases is given to be 30 min = 1/2 hrs

 so ATQ,