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Marble Puzzle

Infront of you is an urn containing an unknown mixture of red, white, and blue marbles. The marbles are thoroughly mixed.
How many blue marbles are there in total?


Let R, W, B be the number of marbles in the appropriate colour. We'll also use M to represent the total number of marbles (R+W+B).
From the first pull, we known that (R - 1)/(M - 1) = 1/9.
Re-arranging this, we determine M = 9R - 8
From the second pull, we know that R/(M - 2) = 1/7.
Substituting M from a above we get 7R = 9R - 8 - 2.
Solving this, we find that R = 5. There are five red marbles.
Plugging this back in we can determine that M = 37. There are 37 marbles in total.
Because we know W = 10, then we can determine that B = 22.
There are 22 blue marbles in the urn.
There are 22 blue marbles in the urn.