Josh had some marbles in his collection. He lost 15 marbles. If he has 10 marbles now How many marbles did he have in his collection?
Josh had some marbles in his collection. 

This is a given, so we'll start with that. 

Josh lost 15 marbles. 

This is also a given, so we'll add that in. 

If he has 10 marbles now, 

This is what we're trying to solve, so we'll leave this blank for now. 

How many marbles did he have in his collection? 

And this is what we're trying to solve, so we'll also leave this blank for now. 

Now, let's think about what we know. We know that Josh had some marbles, and we know that he lost 15. So, if we take away 15 from some, we're left with 10. 

Therefore, Josh had 25 marbles in his collection.