GMAT Quantitative Reasoning Practice Test 3

The new monthly payment should be $1.12\times \mathrm{\$}250=\mathrm{\$}280$. Now we'll proceed with the formula:
Principal $\times$ percent interest $=$ interest eamed
Solve for the principal for the $9\%$ interest: $\left(250\times 12\right)∕.08=\mathrm{\$}37,500$, this is what we invested before.
Solve for the principal for the $6.5\%$ interest: $\left(280\times 12\right)∕.075=\mathrm{\$}44,800$, this is what we should invest with the new interest. $\left(44,800-37,500=\mathrm{\$}7,300\right)$ is the amount to be added to the prime payment.