what does 4/5 of 25 mean?I don't get that

it means you will multiply 4/5 by 25,, thus

4/5 * 25
note that we can rewrite 25 as 25/1 (since any number divided by 1 is equal to itself),, rewriting,
4/5 * 25/1
to multiply fractions, we multiply numerator by numerator and denominator by denominator. note: numerator is the number at the top and denominator is the number below. thus multiplying,
4/5 * 25/1 = (4*25)/(5*1) = 100/5 = 20

hope this helps~ :)