I'll quote the problem from my past papers...
bernard and keith are having a race. bernard drives his car at an avarge speed of 200 km/h and keith at an avarage speed of 160 km/h
after 1 1/2 hours bernard finishes the race
(a) calculate the distance travelled by bernard in this time
(b) how long does it take keith to finish the race?
... and also what is the "trick" of those problems ??