Josh had w marbles in his collection. He lost x marbles and found y new ones. How many marbles does he have now?
First, we need to find out how many marbles Josh had in his collection to begin with. We know that he had w marbles, and we know that he lost x marbles. This means that he must have had w - x marbles to begin with. 

Now, we need to find out how many marbles Josh has now. We know that he started with w - x marbles, and we know that he found y new ones. This means that he must have w - x + y marbles now. 

Therefore, the answer (symbolic expression) is: 

w - x + y