You can make two equations here. First, you know the total number of marbles is 103, so:
![](https://i0.wp.com/66.media.tumblr.com/24a88fabb1f6e2d4bc09442ef0d3a9c0/tumblr_inline_pg9gicnVN81qio24w_540.png?w=852&ssl=1)
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 – 2/5b = 3/5b 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:
![](https://i0.wp.com/66.media.tumblr.com/01e8170839253e1a7f0688b48da78dee/tumblr_inline_pg9gnuuEr81qio24w_540.png?w=852&ssl=1)
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:
![](https://i0.wp.com/66.media.tumblr.com/d6d17b1b50379674ab9082c3816249d7/tumblr_inline_pg9grb0mfp1qio24w_540.png?w=852&ssl=1)
Now substitute that into the second equation and solve:
![](https://i0.wp.com/66.media.tumblr.com/d05fca9bbb7b9695409fddcea057e7b6/tumblr_inline_pg9gztPKis1qio24w_540.png?w=852&ssl=1)
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