Walt made an extra $9000 last year from a part time job. He invested part of the money at 8% and the rest at 9%. He made a total of $750 in interest. How much was invested at 9%?

Answer 1

Let a = the fraction of the $9000 invested at 8% and b = the fraction of the $9000 invested at 9%, A = the amount total at the end of the investment period

The total amount he has at the end of the year is $9750

For the 8% investment A₁ = 9000a(1.08) = 9720a

For the 9% investment A₂ = 9000b(1.09) = 9810b

Then, A₁ + A₂ = 9750 = 9720a + 9810b

and 1 = a + b

We find at a = 2/3 and b = 1/3

So, $6000 is invested at 8% and $3000 is invested at 9%