Two boats A and B start from two points located at a distance of 135 km. towards each other. Speed of boat A and B is 12 km/hr and 15 km/hr respectively. If A is going down stream and B is going upstream, then after how much time will they meet? 

5 hours

Let speed of stream = y km/hr

Speed of A in down stream = 12 + y

Speed of B in up stream = 15 - y

Relative speed = (12 + y) + (15 - y) = 27 km/hour

Meeting time = \(\frac{135}{27}\) = 5 hours