What are two ways to solve this problem?

Question

Carter has a large collection of marbles. He knows that he has exactly 20 blue marbles in the collection, but he does not know how many marbles he has in all. Carter mixes up his marbles and randomly picks out 40 marbles. Of the 40 marbles he picked, four are blue. Based on these results, what is the best estimate you can give for the total number of marbles in Carters collection? Solve the problem in two ways other than the probability formula, explaining your reasoning in each case.

Answer 1

4 : 40 :: 20 : x

4x = 800

x = 200

1 in 10 are blue

20 in 200 are blue