Joan is preparing sandwiches for a family potluck. She decides to make ham sandwiches and grilled cheese sandwiches. One ham sandwich requires 2 slices of cheese, and one grilled cheese sandwich requires 3 slices of cheese. She uses a total of 50 slices of cheese to make the sandwiches. If she makes 10 ham sandwiches, how many grilled cheese sandwiches does she make?
Please reason step by step, and put your final answer within \boxed{}.
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