Time taken by the first coil = $t_1 = \frac{H}{P_1}$

Time taken by the second coil = $t_2 = \frac{H}{P_2}$

H is the amount of heat taken to boil the given amount of water.

Since two coils are in series equivalent Power is P = $\frac{P_1P_2}{P_1+P_2}$

Time taken $T = \frac{H}{P} = \frac{H}{\frac{P_1P_2}{P_1+P_2}}=\frac{H}{P_1}+\frac{H}{P_2}=t_1+t_2 = 35 min$