The -2/5 means we go to the left 2/5 of a full unit. So we take 2 small little steps (going over 2 little tickmarks), then we move 4 small steps to the right due to the +4/5 portion. Ultimately, we end up at 2/5 as the answer
So -2/5 + 4/5 = 2/5
This can be thought of as -2+4 = 2, then you just stick 5 in the denominator for each term.
