Question

It takes 4 minutes to boil a quantity of water electrically, How long will it take to boil same quantity of water using the same heating coil but with the current doubled. Neglect any external heat losses

Answer 1

$$Q=Pt=P't'=I^2Rt=I'^2Rt'\\t'=\frac{I^2}{I'^2}t=\frac{1^2}{2^2}4=1\ min$$