A (x) percentage of a certain number means that if we have 100 persons (x) of them will have this percent. for example if we have a 100 dollar and you have 50% of it. it means if we divide the $100 on 100 persons so everyone of them will take $1. And if we gave you your percentage which is 50% it means that u will take 50 times that each one of the 100 has. which means we have to multiply 50 by their 1$ so your share is $50. so what we basically did is that we took the $100 and multiplied them by 50 and divided them on 100 and the result will be $50.
by the same way
number of parents = 60* total number of attendance/100 =60*20/100= 12