If you do the proportion, you have to put 1/5 = 20/100. The 20 represents the percentage. Then you have to say to yourself that how many time 5 goes to 100, well its x20 then if we multiply 1 x 20 we'll get 20. So, he is correct, 1/5 = 20%. Remember what number we think of when we hear the word percent, the number is 100.
