$A$ man can swim with a speed of $4.0 \; km/h$ in still water. How long does he take to cross a river $1.0 \; km$ wide if the river flows steadily at $3.0 \; km/h$ and he makes his strokes normal to the river current? How far down the river does he go when he reaches the other bank?

(A) Speed of the man in still water,$v_m = 4.0 \; km/h$.
Width of the river,$d = 1.0 \; km$.
Since the man swims normal to the river current,his velocity component across the river is $v_m = 4.0 \; km/h$.
Time taken to cross the river,$t = \frac{d}{v_m} = \frac{1.0 \; km}{4.0 \; km/h} = 0.25 \; h$.
Converting to minutes,$t = 0.25 \times 60 = 15 \; min$.
Speed of the river,$v_r = 3.0 \; km/h$.
The distance covered down the river (drift) is $x = v_r \times t = 3.0 \; km/h \times 0.25 \; h = 0.75 \; km$.
Converting to meters,$x = 0.75 \times 1000 = 750 \; m$.