Question 971516
100 ml *(.40) citric acid =40 ml pure citric acid.

500 ml of 2% citric acid is 500 ml (.02) citric acid and is 10 ml pure citric acid

He needs 25 ml of the 40% citric acid and enough water to make 500 ml.

He is diluting it from 40% to 2%, which is 20 fold.

He needs 20 fold water.

500 ml/25 ml =20 fold dilution.