A dairy has 20 litres of a mixture that is 60% milk. How many litres of a mixture that is 90% milk must be added to get a mixture that is 80% milk?

Explanation

1. 01

   $60\% : 90\% = (90 - 80) : (80 - 60) = 10 : 20 = 1 : 2$.

2. 02

   So the 90% mixture must be twice the 60% mixture.

3. 03

   Needed = $2 \times 20 = 40$ litres.