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EXAMPLE 11  A heater is designed to operate with a power of 1000 W in a 100 V line. It is connected in combination with a resistance of 10 $\Omega$ and a resistance R, to a 100 V mains as shown in the figure. What will be the value of R so that the heater operates with a power of 62.5 W?                                                                                                (JEE 1978)