well, a WHOLE is 1, which we can split in many fractions, say 5/5, 10/10 or 999/999 and so on.
we know 1/5 of the jar is chocolate chip, and there's the rest, well, the Whole jar in 5ths have to be 5/5, if we take away 1/5 from 5/5, the "rest" is 5/5 - 1/5 = 4/5.
now, we also know that 1/2 of the "rest" is peanut butter, or namely that 1/2 of 4/5 is peanut butter, how much will that be? let's divide 4/5 by 2