To travel 648km,an Express train take 12hours more than Rajdhani. if however the speed of the Express train is doubled, it takes 6hours less than rajdhani. the speed of rajdhani is....
1)36
2)18
3)45
4)27 km/h

speed of the express train=distance÷(time taken by raajadhani +12)
S=648÷(RT+12)...................(1)
if speed of express is doubled then
2s=648÷(RT-6).....................(2)
by solving both equations we get time taken for raajadhani train to travel the distance of 648 kms.
by substituting the value of RT in above equations we get speed of raajadhani Tarin= 27 kmph.