Explanation

(c) In first case
$\frac{m \times s \times (61{^\circ} - 59{^\circ})}{4} = K\left\lbrack \left( \frac{(61{^\circ} - 59{^\circ})}{2} \right) - 30{^\circ} \right\rbrack$ …(i)
In second case
$\frac{m \times s \times (51{^\circ} - 49)}{t} = K\left\lbrack \left( \frac{(51{^\circ} - 49{^\circ})}{2} \right) - 30{^\circ} \right\rbrack$ …(ii)
Dividing Eq.(i) by Eq.(ii), we obtain
$\frac{t}{4} = \frac{30}{20} = \frac{3}{2}\ \ \ \ \ \ or\ \ \ \ t = 6\ \min$