What is the difference between the simple interest on a principal of Rs. 500 being calculated at 5% per annum for 3 years and 4% per annum for 4 years?

$$\eqalign{ & {I_1} = \frac{{P{T_1}{R_1}}}{{100}} \cr & {I_1} = \frac{{ {500 \times 3 \times 5} }}{{100}} \cr & \,\,\,\,\,\,\, = Rs.{\kern 1pt} 75 \cr & {I_2} = \frac{{P{T_2}{R_2}}}{{100}} \cr & {I_2} = \frac{{ {500 \times 4 \times 4} }}{{100}} \cr & \,\,\,\,\,\,\,\, = Rs.{\kern 1pt} 80 \cr & {\text{Difference}} = 80 - 75 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{.}}\,5 \cr} $$

Alternatively
The interest is calculated simply and then it will have a rise of 15% in 1^st case and 16% in 2^nd case.
Difference = 1% on 500 = Rs. 5

Otherway,
500 == 15% $$ \uparrow $$ ⇒ 575 (1^st case)
500 == 16% $$ \uparrow $$ ⇒ 580 (2^nd case)
We can see clear difference of Rs. 5