Josh had some marbles in his collection. He lost 15 marbles. If he has 3 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 3 marbles now, 

This is our question. We're looking for the number of marbles Josh had in his collection. 

How many marbles did he have in his collection?

This is what we're looking for, so we'll solve for this. 

We know that Josh had some marbles in his collection, and we know that he lost 15 marbles. So, we can say that Josh had 15 + 3 = 18 marbles in his collection.