P and Q are 18 Kms. away.  Two trains with speeds of 20 km/h and 15 km/h respectively start simultaneously from P and Q and travel in the same direction.  They meet at a point R beyond Q is?

54 Km

                    1st    :    2nd   

Speed            20    :    15

                       4    :     3

Distance           4    :     3

                        ↓          ↓

                           1R → 18

                           3R → 54

Distance = QR = 54 Km