How many liters of water should be added to a 30 liters mixture of milk and water containing milk and water in the ratio of 7 : 3 such that the resultant mixture has 40% water in it?

Question: How many liters of water should be added to a 30 liters mixture of milk and water containing milk and water in the ratio of 7 : 3 such that the resultant mixture has 40% water in it?

Solution:
Water in the mixture = 30 × (3/10) = 9 liters

ATQ,
9 + x = 40% of (30 + x)
⇒ 9 + x = (2/5)(30 +x)
⇒ 45 + 5x = 60 + 2x
⇒ 3x = 15
⇒ x = 5