Explanation

(b) Let $v_{w}$ be velocity of water and $v_{b}$ be the velocity of motor boat in still water. If $x\ $is the distance covered, then as per question
$x = \left( v_{b} + v_{w} \right) \times 6 = (v_{b} - v_{w}) \times 10$
On solving, $v_{w} = v_{b}/4$
$\therefore\ x = \left\lbrack v_{b} + v_{b}/4 \right\rbrack \times 6 = 7.5v_{b}$
Time taken by motor boat to cross the same distance in still water is
$t = \frac{x}{x_{b}} = \frac{7.5}{v_{b}} = 7.5$h