this is because one car is moving at 30 miles an hour and the other at 30+20 miles an hour = 50mph. so after 1 hour one car would have gone 30 miles and the other 50 = 80 altogether not 400 yet. After 2 hours one car would have gone 60 miles and the other 100 = 160 not 400 yet. ECT... After five hours one car has gone 150 mile (30mph*5hr = 150miles) and the other 250miles (59mph*50hr = 250miles) = 400, since 150+250=400. Therefore it would take 5hr for the cars to meet. and crash ._.
