Solution:
Efficiency \(\eta = \frac{P_{\text{out}}}{P_{\text{in}}} \times 100% = \frac{50}{220 \times 1/4} \times 100% = \frac{50}{55} \times 100% \approx 91%\).
A transformer connected to \(220\text{ V}\) mains is used to light a bulb of rating \(50\text{ W}\) and \(110\text{ V}\). If the primary current is \(\frac{1}{4}\text{ A}\), the efficiency of the transformer nearly is
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