A transformer is used to light a $140$ watt, $24$ volt lamp from $240 V$ AC mains. The current in the main cable is $0.7$ amp. The efficiency of the transformer is

48

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63.8

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83.3

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90

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Solution

The correct option is C $83.3$ %
Mains voltage is $240 V$ and mains current is $0.7$ amp so inpur power will be $240 \times 0.7 = 168 w a t t$
But output power is given as $140$ Watt so efficinecy $= (o u t p u t / i n p u t) \times 100$ % $= 83.33$ %
Option C is correct.

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