At simple interest, a sum doubles itself in 8 years. The annual rate is:

1. Doubling means the interest earned equals the original principal. 2. Take the principal as 100, so the interest over 8 years must also be 100. 3. Simple interest gives 100 x R x 8 / 100 = 100. 4. So 8R = 100, giving R = 12.5. 5. The rate is 12.5 per cent per annum. Note this only works for simple interest; under compounding the sum would double sooner. _Source: NCERT Class 8 Maths, Ch 7 'Comparing Quantities', S7.4 Compound Interest_