The solution:

Refer Figure 1. Let M be the man and T be the temple on the opposite bank of the river. The man wants to reach T but the river is flowing at a speed of 3m/s. Due to the flow of the river, the man reaches point P (i.e., he drifts slightly off-course). Now the time taken by the man to cross the river can be calculated by considering the direction MT only (using vectors) = MT/ 4 = 800/4 =200 sec .... (1) In 200 sec, the man drifts to point P, so the distance PT = 3 * 200 = 600 m .... (2) Now the man walks at a speed of 2 m/s, so the time taken by him to travel from P to T = PT/ 2 = 600/2= 300 sec .... (3) So the total time required to reach T = eqn 1 + eqn 3 = 200 + 300 = 500 seconds.