>>12472027>>12471995To simplify what he said for you
The red triangle has a slope of 3/8, the blue triangle has a slope of 2/5. Or 15/40 and 16/40 respectively. Since the difference is so small (1/40) it is easy to not notice it.
Consequently, the area of the upper left white area changes from (2*8) + (8*3/2) + (2*5/2) to (3*5) + (8*3/2) + (2*5/2), or from 33 to 32. The difference here is 1, which is the white square between green and yellow in the end state
