A foundation invests ​$80 comma 000 at simple​ interest

Question

A foundation invests $80 comma 000 at simple interest, a part at 7%, twice that amount at 3%, and the rest at 5.5%. What is the most that the foundation can invest at 3% and be guaranteed $4120 in interest per year?

Answer 1

Principle * Interest = profit

x*.07 = .07x

2x*.03 = .06x

(80000-3x)*.055 = 4400-.165x

Total should be 4400-.015x = 4120.

.015x = 280

x= 18666.67

2x=37333.33