Johny is a professional boxer. He has played 100 fights of which he has won 85, has lost 10 and has drawn five. He wants to retire with 90% of the fights won.
How many more fights (at least) should he play?
85 + 10 + 5 make a total of 100 fights so their current percentage of fights won is 85%.
If you want to reach up to 90% it will be necessary for the number of defeats + draws to be 10% of the total, so the sum of both multiplied by ten will be the answer. 15 * 10 = 150 is to say that you must fight and win the next 50 fights.