How many litres of a 30% alcohol solution should be added to 40 litres of a 60% alcohol solution to prepare a 50% solution?

Question: How many litres of a 30% alcohol solution should be added to 40 litres of a 60% alcohol solution to prepare a 50% solution?

Solution:
Let,
x litres of 30% alcohol solution be added.

ATQ,
30% of x + 60% of 40 = 50% of (x + 40)
⇒ (30x/100) + (60 × 40)/100 = 50 × (x + 40)/100
⇒ (30x + 2400)/100 = (50x + 2000)/100
⇒ 30x + 2400 = 50x + 2000
⇒ 50x - 30x = 2400 - 2000
⇒ 20x = 400
∴ x = 20