160 is the answer.
You know that 20%, or 1/5 equals 64 so you multiple 64 by 5 to get the full 100% number. In this question 320. You then divide 320 by 100 to get the value per 1%. Then multiple this by 50 to get 160.
To check that you have the right answer, you divide 320 by 100 then multiple by 20 and if you get 64, you have the right answer.
