The voltage across the secondary winding is 10,000 V, and the voltage across the primary winding is 25,000 V. If the primary winding has 200 coils, how many coils are in the secondary winding?

For an ideal transformer, the ratio of secondary to primary voltage equals the ratio of secondary to primary coil turns: 10,000 ÷ 25,000 = Ns ÷ 200. Thus Ns = 200 × (10,000 ÷ 25,000) = 80. The secondary winding has 80 coils, which is choice D.

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