You can make two equations here. First, you know the total number of marbles is 103, so:

The second equation is more complicated, so let’s do it in parts. First, he gives away 15 red marbles, so he should have *r* – 15 left. He gives away 2/5 of his blue marbles, so he should have *b *– 2/5*b* = 3/5*b *left.

So the ratio of red marbles he has left to blue marbles he has left (which the question tells us is 3/7) should be:

The question asks how many blue marbles he had originally, so let’s substitute and solve for *b*. First get *r* by itself in the first equation:

Now substitute that into the second equation and solve:

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