400 rupees were lent by a person to a buddy at a rate of 4% annually. After six months, he gave the money back. What was the cost?

Given, Principal amount P = Rs 400

Time period T = $\frac{1}{2}$ year

Rate of interest R = 4% p.a.

Simple interest = $\frac{\mathrm{P} \times \mathrm{T} \times \mathrm{R}}{100}$

SI = $\frac{400 \times {\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2 $}\right.}\times 4}{100}$

SI = Rs 8

Total amount paid after $\frac{1}{2}$ year = Principal amount + Interest

= Rs 400 + Rs 8

= Rs 408