Your question answers itself. Marty Lister invested $9000.
Perhaps you want to know how much Marty invested in each asset type. We can let b represent the amount in bonds. Then the total interest is given by
5%(9000-b) + 8%(b) = 525
3%(b) + 450 = 525 (use the distributive property, collect terms)
b = 75/.03 (subtract 450, divide by 3%)
b = 2500
Perhaps you want to know how much Marty invested in each asset type. We can let b represent the amount in bonds. Then the total interest is given by
5%(9000-b) + 8%(b) = 525
3%(b) + 450 = 525 (use the distributive property, collect terms)
b = 75/.03 (subtract 450, divide by 3%)
b = 2500
