Josh had some marbles in his collection. He lost 21 marbles. If he has 12 marbles now how many marbles did he have in his collection?
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Josh had some marbles in his collection. We'll call this number of marbles "x".

He lost 21 marbles. This means that x - 21 = 12.

We can solve this equation for x by adding 21 to both sides. This gives us x = 12 + 21, or x = 33.

So, Josh had 33 marbles in his collection.