Sum Rs. 20000 and Rs. 40000 are given on simple interest at the rate of 10% and 15% per annum respectively for three years. What will be the total simple interest?

Answer: Option C

Simple interest on 20000 for 3 years at 10% rate
$$\text{SI}=\frac{20000\times 3\times 10}{100}=6000$$
Simple interest on 40000 for 3 years at 15% rate
$$\text{SI}=\frac{40000\times 3\times 15}{100}=18000$$
Total interest = 6000 + 18000 = 24000