The ratio of speed of a boat in still water to speed of current is 7:3 respectively.   Boat can cover 112 km of still water in four hours. If the boat can cover X km upstream in two hours, then find the time (in hour) taken by boat to cover X+16 km downstream.

Correct option is A

​$$\text{Speed of a boat in still water \& speed of current is } 7s \text{ and } 3s \text{ respectively}\\[10pt]\frac{112}{7s} = 4\\[5pt]s = 4\\[10pt]\text{Upstream speed} = 7s - 3s = 4s\\[5pt]4 \times 4 = 16 \text{ km/hr}\\[10pt]\text{Downstream speed} = 7s + 3s = 10s\\[5pt]10 \times 4 = 40 \text{ km/hr}\\[10pt]\text{ATQ,} \quad \frac{X}{16} = 2\\[5pt]X = 32\\[10pt]\text{Required time} = \frac{32 + 16}{40} = 1.2 \text{ hours}$$​​