The solution comes in the form of a simple equation
3/8+1/4+x=7/8 because if you used 7/8ths you would only have 1/8th left
you need common denominators to solve so you find the closest one which is 8
you have to times 4 by 2 to get 8 and whatever you do to the 4 you have to do to the one above it so 12=2 and 42=8 so back to the equation 3/8+2/8+x=7/8
add them together and you have 5/8+x=7/8 so... 7/8-5/8=2/8ths
He used 2/8ths to paint the birdhouse
