Question 1036070
Call the number he completed, x.  Then the number he didn't complete would be 40-x.  Do you see that?  If you do, we then write
20x - 8(40-x) = 380
20x - 320 + 8x = 380
28x - 320 = 380
28x = 700
x = 700/28 = 25 bird houses he completed