Well, we know the total amount of students in the class is 20. 4 of them prefer SS, and 75% of them prefer PE.
now, if we take 20 to be the 100%, what is 4 off of it in percentage?

so, we know 20% prefer SS, and 75% prefer PE, well, notice the percentages, is clear that 20% < 75%
if we make the total class say hmmm "x", then 20% of x is (20/100) * x, or 0.2x, and 75% of x is (75/100) * x, or 0.75x, thus