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An electric kettle has two heating coils. Used alone, the first boils the water in 15 minutes and the second boils it in 30 minutes. If both coils are connected in series and switched on, the water boils in alpha/4 hours. Assuming no heat loss, find the value of alpha.

1. 2
2. 3
3. 4
4. 5

Correct answer: 3

Solution

Since t is proportional to R at constant voltage and heat, the series combination takes t1 + t2 = 15 + 30 = 45 min = 3/4 hour, so alpha = 3.

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