
speed and time problems
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 ??

Speed is a measure of distance in a certain time, or distance divided by time. With this, you can get an equation relating the three, like speed = distance/time. If you rearrange this equation, it can become distance = speed*time, or time = distance/speed.
(a) calculate the distance travelled by bernard in this time
You know speed and time, so you can plug them into the first equation to find distance, so:
distance = 200*1.5 = 300km
(b) how long does it take keith to finish the race?
You now know distance and speed, so you can find time with the second equation:
time = 300/160 = 1.875 hours
You have to be careful when you use these equations, though, and make sure that the units of time and speed match up. Since the speed was km per hour, and our time was hours, we were ok.