SSC CHSL 6 Dec 2015 Shift 1 Paper

Let speed of boat = x km/hr
⇒ Downstream speed = $\left(\mathit{x}+1.5\right)$ km and upstream speed = $\left(\mathit{x}-1.5\right)$ km
∵ Distance travelled is same and speed is inversely proportional to time,
⇒ $\frac{\mathit{x}+1.5}{\mathit{x}-1.5}=\frac{3.5}{3}$
⇒ $3\mathit{x}+4.5=3.5\mathit{x}-5.25$
⇒ $3.5\mathit{x}-3\mathit{x}=4.5+5.25$
⇒ $\frac{\mathit{x}}{2}=9.75$
⇒ $\mathit{x}=9.75\times 2=19.5$
∴ Speed of the boat in still water = 19.5 km/hr