1) The speed of motorboat in still water is 6 km/h. A motor boat goes 7 km upstream and back to the starting point in $2\frac{2}{5}$ hrs. The velocity of the current is A) 1.2 km/h B) 3 km/h C) 1.5km/h D) 1.3 km/h E) 1 km/h Answer: Option EExplanation:Let velocity of current $=x$ km/h Upstream speed $=(6–x)$ km\/h Downstream speed $=(6 + x)$ km/h $7\times\left(\frac{1}{6-x}+\frac{1}{6 + x}\right)$ $=\frac{12}{5}$ $x$ = 1, -1 $x$ = 1 km/h (negative cannot be speed)
