A mixture of 150 liters of Petrol and Diesel contains 20% Diesel. How much more Diesel should be added so that Diesel becomes 25% of the new mixture?

Let us assume P liters of Diesel added to the mixture so that Diesel will be 25% in the new mixture.
According to Question,
Quantity of Diesel in 150 liters of the mixture = 20% of 150 = 150 x 20/100 = 30 liters.
After adding P liters of Diesel, The total quantity of Diesel becomes (30 + P) and total volume of mixture will be (150 + P).
Again According to Question,
After adding the P liters of Diesel in the mixture, the Diesel quantity becomes 25% of the new mixture.
Quantity of Diesel in new mixture = 25% of Total mixture.
⇒ (30 + P) = (150 + P) x 25 %
⇒ 30 + P = (150 + P) x 25/100
⇒ 30 + P = (150 + P) x 1/4
⇒ 120 + 4P = 150 + P
⇒ 4P - P = 150 - 120
⇒ 3P = 30
⇒ P = 10 liters.