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An electric kettle has two heating coils. When one of the coils is connected to an a.c. source, the water in the kettle boils in 10 minutes. When the other coil is used, the water 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

1. 15 min
2. 8 min
3. 4 min
4. 25 min

Correct answer: 8 min

Solution

The power of a heating coil is inversely proportional to the time taken to boil the water. If the first coil takes 10 minutes and the second takes 40 minutes, their powers are in the ratio 4:1. When connected in parallel, the total power is the sum of individual powers, i.e., 5 units. The time taken is inversely proportional to the total power, so the time is (1/5) of the time taken by the slower coil, which is 8 minutes.

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