The difference between simple interest and the true discount on Rs. 2400 due 4 years hence at 5% per annum simple interest is -

$$\eqalign{ & {\text{Given}}\,{\text{A}} = 2400 \cr & {\text{R}} = 5\% \cr & {\text{T}} = 4{\text{year}} \cr & \therefore \,TD\, = \frac{{A \times R \times T}}{{100 + R \times T}} \cr & TD = \frac{{2400 \times 4 \times 5}}{{100 + 20}} \cr & TD = {\text{Rs}}{\text{. }}400 \cr & \therefore \,{\text{S}}{\text{.I}}{\text{.}} - {\text{TD}} = \frac{{TD \times R \times T}}{{100}} \cr & = \frac{{400 \times 5 \times 4}}{{100}} \cr & = {\text{Rs}}{\text{. 80}}{\text{}} \cr} $$