List of top Quantitative Ability Questions on Relative speed equations

Two trains 'A' and 'B' started running at the same time. Train 'A' takes 3 hours more than train 'B' to cover 540 km. But if train 'A' doubles its speed, it covers 540 km in 1½ hours less than train 'B'. What is the speed (in km/h) of train 'B'?