The combined yearly interest for x dollars invested at 13% and 20,000-x dollars invested at 8% is

total = 1600 + 0.05x

Step-by-step explanation:

I = Prt

You want the sum of the interest from the two investments. The interest at 13% is ...

I = Prt = x (0.13) (1) = 0.13x

The interest at 8% is ...

I = Prt = (20000-x) (0.08) = 1600 - 0.08x

Then the total interest earned is ...

total = 0.13x + (1600 - 0.08x)

total = 1600 + 0.05x