Question=> Find the difference between the interest earned from S1 for the 2nd year and from S2 for the 3rd year.
Statement 1=> At the beginning of year 2, S1 amounts to $525. This means that principal + interest at the end of year 1 = $525
The exact value of rate of S2 is unknown, hence, statement 1 is NOT SUFFICIENT.

Statement 2=> At the end of year 1, S2 earns $25 more interest compared to S1.
Let interest for S1 and S2 = r1 and r2. $$\left(500\cdot\frac{r2}{100}\right)-\left(500\cdot\frac{r1}{100}\right)=25$$
The exact values of r1 and r2 is unknown, hence, statement 2 is NOT SUFFICIENT

Combining both statement together;
$$\left(500\cdot\frac{r2}{100}\right)-\left(500\cdot\frac{r1}{100}\right)=25$$
$$5r2-5r1=25\ \left(divide\ all\ through\ by\ 5\right)$$
$$r2-r1=5$$
Since, r1=5
r2 = 5 + 5
There both statements combined together ARE SUFFICIENT