How many litres of pure alcohol must be added to 10 litres of mixture which is 15% alcohol to make a mixture which will be 25% alcohol?

⇒ Let x liters of pure alcohol added to mixture

⇒ Quantity of alcohol in 10 liters of mixture = 15% of 10 = 1.5 litre

⇒ (10 + x) x 25% = (1.5 + x)

⇒ 2.5 + 0.25x = 1.5 + x

⇒ 1 = 0.75x

∴ x = 4/3