20. There is a sum of money, used to buy a notebook for each student in Class 4 (1). If each notebook costs 3 yuan, then 6 more can be bought; if each notebook costs 5 yuan, then 30 yuan is short. If the money is used up, just enough to buy a notebook for each student, then a total of $\qquad$ notebooks are bought, among which 3-yuan notebooks are $\qquad$ in number.
【Analysis】If each book costs 3 yuan, then there will be an excess of $3 \times 6=18$ yuan, so the total number of people is $(18+30) \div(5-3)=24$ people, and the total amount of money is $5 \times 24-30=90$ yuan, from which we can draw the conclusion.

【Solution】From the problem, if each book costs 3 yuan, then there will be an excess of $3 \times 6=18$ yuan, so the total number of people is $(18+30) \div(5-3)=24$ people, and the total amount of money is $5 \times 24-30=90$ yuan,

If the money is just enough to buy 24 books, then the number of notebooks costing 3 yuan each is $(24 \times 5-90) \div(5-3)=15$, so the answer is 24, 15.

【Comment】This problem tests the allocation and profit/loss problem, examining students' computational skills, and is considered a medium-level question.