Two coils require 20 minutes and 60 minutes respectively to produce...

If $H is the heat required to heat the water, then</p><p>${P}_{1} \times 60 = H ⇒ {P}_{1} = \dfrac{H}{60} and {P}_{2} \times 20 = H ⇒ {P}_{2} = \dfrac{H}{20}$</p><p>When the coils are connected in parallel their powers will get added. If the required time now is$t$, then</p><p>\left({P}_{1} + {P}_{2}\right)t = H ⇒ t = \dfrac{H}{\dfrac{H}{60} + \dfrac{H}{20}} = 15min$