On Fri, 5 May 2017 09:54:31 -0700 (PDT), wrote
in

I only want to mix a cup of gas at 40:1 how much oil will it take

1 cup of gas = 8 oz = 1/16 of a gallon

if the container of oil you have is meant to be entirely mixed with a
gallon, take 1/16 of that oil and mix it with the gas. Or use 8/40 =
0.2 oz of oil for a 40:1 mix.
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