I need help getting the answer to this

Question

a foundation invests 75000 at simple interest a part at 7% twice that amoiunt at 3% and the rest at 6.5% what is the most that the foundation can invest at 3% and be guaranteed 4485 in interest per year?

Answer 1

Let x = amount invested at 7%

Then, 2x = amount invested at 3% and 75000 - x - 2x = 75000 - 3x = amount invested at 6.5%

0.07x + 0.03(2x) + 0.065(75000 - 3x) = 4485

-0.065x + 4875 = 4485

-0.065x = -390

x = 6000

Invest $6000 at 7%, $12000 at 3%, and $57000 at 6.5%