A sum doubles in 10 years at simple interest. Find its rate of interest.
10%
Let the principal be P. If the sum doubles, the amount becomes 2P, so the simple interest earned = 2P − P = P.
Simple interest formula: \(SI = \dfrac{P \times R \times T}{100}\), with T = 10 years.
So \(P = \dfrac{P \times R \times 10}{100}\), which gives \(10R = 100\), i.e. R = 10.
Hence, the rate of interest is 10% per annum.
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