Method for this problem?

Question

I need help solving this problem (not the answer, just the process):

Bill invests in a plan that has an APR of 3%. He invests four times as much in a plan that has an APR of 8%. If the total interest from the investments is $1400 after one year, how much was invested in each plan?

Answer 1

Let x = the amount at the APR of 3%, then 4x = the amount at 8%.

The interest earned is

.03(x) + .08(4x) = 1400

Solve for x.

.03x + .32x = 1400

.35x = 1400

x = 1400/.35

x = 4000 at 3% APR

4(4000) = 16000 at 8% APR.