To do this kind of problem, we should find an invariant as "1". Because the bucket is unchanged, it is considered as "1"

Then crude oil accounts for 1/(8%) = 25/2 barrels, and late oil accounts for 1/2 barrels.

The barrel weight is 48/(25/2- 1/2) = 4 (kg).

So crude oil 4/(8%) = 50 (kg)