The cafeteria used 100 lasagnas * 2 pounds of ground mince = <<100*2=200>>200 pounds of ground mince for the lasagnas.
This means they used 500 pounds of ground mince in total – 200 pounds of ground mince for the lasagnas = 300 pounds of ground mince for the cottage pies.
So they must have made 300 pounds of ground mince for the cottage pies / 3 pounds of ground mince per cottage pie = <<300/3=100>>100 cottage pies.
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