on a journey of 250 km, a driver calculated that by increasing his average speed, by 5 km/h, he would take 25 minutes less. find his usual average speed , correct to 2 dec. places.

Ans: 52.33 km/h

i really need to know how to get this answer- i've tried by forming the quadratic equation:

250-250=25

x (x+5) 60

by keeping the speed as x.

pls help!