Explanation:
as 8.5% of such a barrel are turned into plastic, that means 100 - 8.5 = 91.5% of such a barrel are not turned into plastic.
remember, 100% means "the whole".
therefore, any % number is always an expression of that many 1/100 of the whole.
and so,
42 × 91.5/100 = 42 × 0.915 = 38.43 gallons
out of the 42 gallons, 38.43 gallons are not turned into plastic.