You have .12 and .08, the two percentage interests. You also know that at the end you get 1800 from your starting 20,000. One part, let's call it x, is put at .12 interest. The other part is put at .08 interest. We know this part is whatever we didn't put into x, so it is 20,000-x. Write this all out

.12(x) + .08(20,000-x) = 1800. Clear the decimals.

12x +8(20,000-x) = 180,000 Multiply out

12x + 160,000 -8x = 180,000 Combine the terms

4x = 20,000. Divide by 4

x = 5,000.

So you put 5,000 at 12% interest, and (20,000-5,000)= 15,000 at 8% interest.