If a sum of money double itself in 20 years. What is the

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If a sum of money double itself in 20 years. What is the rate of simple interest per cent per year ?

Let the principal = Rs. P
⇒ Amount = Rs. 2P
⇒ Interest = 2P - P = Rs.P
∴ Rate = (interest x 100) / (Principal x 20) = (P x 100) / (P x 20) = 5%