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, $v_w=v_b/4$

$\therefore x=\left[v_b+v_b/4\right]\times 6=\text{7.5}{v}_b$

Time taken by motor boat to cross the same distance in still water is

  $t=\frac{x}{v_b}\frac{\mathrm{7.5 }{v}_b}{v_b}=7.5\mathrm{h}$