An electric kettle has two coils. When one of these is switched on, the water in the kettle boils in 6 minutes. When the other coil is switched on, the water boils in 3 minutes. If the two coils are connected in series , find the time taken to boil the water in the kettle .

Question

$\begin{array}{1 1} 9\;m \\ 10\;m \\ 100\;s \\ 10\;s \end{array} $