A train is 1000 meters long. It is travelling at a constant speed, and approaches a tunnel that is 3000 meters long. From the time that the last car on the train has completely entered the tunnel until the time when the front of the train emerges from the other end, 30 seconds pass. Determine the speed of the train, in kilometres per hour.

If you only know how to multiply with 4's, then you need to quickly convert the 8's to 4's.

We were given,



So we rewrite each of the 8's as



Perfectly for us,it turns out that the 2's will also give us a 4, so the whole thing becomes,





Now, let us multiply two of the 4's, to obtain,



Finally, let us multiply with the last 4 to get,