A mixture of 40 L of alcohol and water contains 10% water. How much water should be added to this mixture so that the new mixture contains 20% water?

From a very early age, I knew that when I grew up, I should be a writer. I had the lonely child's habit of making up stories and holding conversations with imaginary persons, and I think from the very start my literary ambitions were mixed up with the feeling of being isolated and undervalued. I knew that I had a facility with words and a power of facing unpleasant facts, and I felt that this created a sort of private world in which I could get my own back for my failure in everyday life. I wanted to write enormous naturalistic novels with unhappy endings, full of detailed descriptions and arresting similes, and also full of purple passages in which words were used partly for the sake of their sound. I give all this background information because I do not think one can assess a writer's motives without knowing something of his early development.
His subject-matter will be determined by the age he lives in — at least this is true in tumultuous, revolutionary ages like our own — but before he ever begins to write he will have acquired an emotional attitude from which he will never completely escape. It is his job to discipline his temperament, but if he escapes from his early influences altogether, he will have killed his impulse to write. I think there are four great motives for writing, at any rate for writing prose. They are: (i) Sheer egoism: Desire to seem clever, to be talked about, to be remembered after death, to get your own back on grown-ups who snubbed you in childhood; (ii) Aesthetic enthusiasm: Desire to share an experience which one feels is valuable and ought not to be missed (iii) Historical impulse: Desire to see things as they are, to find out true facts and store them up for the use of posterity (iv) Political purpose: Desire to push the world in a certain direction, to alter other people's idea of the kind of society that they should strive after.
[Extracted with edits from George Orwell's "Why I Write"]