What is the simple interest on a sum of Rs. 700 if the rate of interest for the first 3 years is 8% per annum and for the last 2 years is 7.5% per annum?

Answer: Option C

$$\begin{array}{rl}& 1^{\text{st}}\text{case}:\\ & I_1=\frac{700\times 3\times 8}{100}=\text{Rs}\text{. }168\\ & 2^{\text{nd}}\text{case}:\\ & I_2=\frac{700\times 2\times 7.5}{100}=\text{Rs}\text{. }105\end{array}$$
Then total interest for five years
$$=\left(I_1+I_2\right)=\text{Rs}\text{. }273$$

Alternatively,
As interest is calculated as simple interest
So, we can add up rates for all given 5 years and calculate it easily i.e.
For the five years rate
= (8 × 3 + 7.5 × 2)
= 39%
Now,
700 = 39% $$\uparrow$$ ⇒ 973
Interest = Rs. 273
The thought can go this way we internally calculated
$$\begin{array}{rl}& 10\%\text{of}700=\frac{700}{10}=70\\ & \text{Then},\\ & 39\%\text{of}700\\ & =\left(40\%-1\%\right)\text{of}700\\ & =\left(280-7\right)\\ & =273\end{array}$$