Explanation:
The river is 1000 m wide, and the boat moves at 3 m/s, so the time it takes to cross the river is:
t = (1000 m) / (3 m/s)
t = 333.3 s
The river flows at 2 m/s, so by the time he reaches the other bank, he drifts downstream a distance of:
d = (2 m/s) (333.3 s)
d = 666.7 m