If the tramcars leave their terminals every x minutes, that means that x minutes after I have met a tramcar the following car arrives at that spot. If it is catching up with me, then during the remaining 12 - x minutes it has to cover the same distance that I do in 12 minutes. Thus, the distance that I cover in one minute is covered by the tram (12-x)/12 minutes. Now if the tram is coming towards me, then it will meet me 4 minutes after the preceding one, and during the remaining in( (x - 4) minutes it will cover the same distance that I do in 4. minutes. This means that the distance I cover in one minute is covered by the tram in (x-4)/4 minutes. We get the equation --- (12-x)/12 = (x-4)/4 Thus x = 6 .The tramcars start out every 6 minutes.