A drum holds 90 litres of pure blue paint. 9 litres is drawn off and replaced by white paint; the drum is stirred, 9 litres of the mixture is drawn off, and this time yellow paint is added to fill the drum. The blue paint now left in the drum (in litres) is:

Each draw-off removes 9/90 = one tenth of whatever is in the drum, so the blue survives twice at 90%: $90\times 0.9\times 0.9=72.9$ L. (It makes no difference that the second top-up is a different colour — only the blue is being tracked. Taking 9 L of blue away twice gives 72 L, which forgets that the second draw-off was partly white; 81 L stops after the first replacement.)