Final answer:
The maximum amount of purple paint Yan can make is 21 litres.
Step-by-step explanation:
To find the maximum amount of purple paint Yan can make, we need to determine the limiting factor between the amounts of red and blue paint he has.
Since purple paint is made by mixing red and blue paint, we will need an equal amount of each.
Since Yan has 30 litres of red paint and 21 litres of blue paint, he can only make as much purple paint as the smaller of the two amounts.
Therefore, the maximum amount of purple paint Yan can make is 21 litres.