Question 848245
Let x be the amount invested at 3% and (9000-x) be the part invested 3.5%.

.03x + .035(9000-x) >= 300

.3x + 315 - .035x >= 300

-.005x >= -15

x <= 3000

Which means he can put at most $3000 in the 3% to guarantee the return of $300.