MATH Inequalites word problem with three variables

Question

A foundation invests $85,000 at simple interest, part at 7%, twice that amount at 4%, and the rest at 6.5%. What is the most that the foundation can invest at 4% and be guaranteed $5,210 in interest per years.

Answer 1

.07X + .04(2X) +.065(85000 - 3X) = $5210

 

.07X + .08X + 5525 - .195X = $5210    

 

.15X + 5525 - .195X = $5210

 

- .045X = - 315       -.045X/-.045 = -315/-.045           X = 7000       2X = $14000 invested at 4%