An electric kettle has two heating coils. When one of the coils is connected to an a.c. sotuce, the water in the kettle boils in $10$ minutes. When the other coil is used the w’ater boils in $40$ minutes. If both the coils are connected in parallel, the time taken by the same quantity of water to boil will be ...... $min$
A$15$
B$25$
C$8$
D$4$
AIPMT 2003, Medium
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C$8$
c $Q=\frac{V^2}{R_1} \times t_1=\frac{V^2}{R_2} \times t_2=\frac{V^2}{R} \times t$
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