A solution is mixed 3 parts water to 1 part concentrate. How much water is in 20 litres of solution?

Correct answer to question 77 (Arithmetic Reasoning): 15 litres

Method

The mixture divides into 3 + 1 = 4 parts of 5 litres each, and water is 3 of them, giving 15 litres. Using 3/3 instead of 3/4 gives the wrong denominator, which is the usual slip.