20 litres of chemical A are added to 80 litres of mixture which is 10% chemical A and 90% chemical B. What percentage of the resulting mixture is chemical A?

The mixture first contains 10% of 80 L = 8 L of chemical A. Adding 20 L gives 28 L of chemical A in 100 L of mixture, which is 28%. Simply adding the percentages, 10% + 20% = 30%, or taking 20 out of 100 to get 20%, ignores the 8 L already present or misstates the total.