math
posted by lori on .
A professor wants to supplement her pension with investment interest. If she invests $26,000 at 7% interest, how much more would she have to invest at 9% to achieve a goal of $4,160 per year in supplemental income?

What is the interest period? Annually, quarterly, monthly, or what?
Assuming annually. Make adjustments if otherwise.
Supplemental income is interest earned over the period. For one year this is:
4160 = 0.07*26000 + 0.09*x
Solve for x.