A 120-litre drum is full of pure groundnut oil. 24 litres is drawn off and replaced by refined oil; the drum is stirred, and this time 30 litres of the mixture is drawn off and again replaced by refined oil. The groundnut oil left in the drum (in litres) is:

The first draw-off removes 24/120 = one fifth of the contents, leaving $120\times 0.8=96$ L of groundnut oil. The second removes 30/120 = one quarter of whatever is there, leaving $96\times 0.75=72$ L. (The two draws are of DIFFERENT sizes, so the two surviving fractions differ: using 0.8 twice gives 76.8 L and using 0.75 twice gives 67.5 L. Subtracting 24 + 30 = 54 L straight from 120 to get 66 L forgets that the second bucketful was already part refined oil.)