An electric kettle has two coils. When one of these is switched on, the water in the kettle boils in $6\,\min$ . When the other coil is switched on, the water boils in $3\,\min$. If the two coils are connected in series, the time taken to boil the water in the kettle is ............. $min$
A$3$
B$6$
C$2$
D$9$
Medium
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D$9$
d In series $\frac{1}{{{P_S}}} = \frac{1}{{{P_1}}} + \frac{1}{{{P_2}}}$
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