For the unknown number the denominator (total) is 72 and blue is unknown, so call it x. The ratio is x to 72 or x/72.

Create a proportion by setting the ratios equal: 5/8 = x/72.

Cross multiply: 5*72 = 8x ---> 360=8x ---> (divide both sides by 8) 45=x.

There are 45 blue marbles and 27 green marbles in the bag.

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