There were 60 pies before the bake sale started.
<u><em>Explanation</em></u>
Suppose, there were number of pies before the bake sale started.
As Mary set aside 2 pies for herself at the beginning of the bake sale, so the <u>remaining number of pies</u>
Karen then bought half of the remaining pies for her birthday party. That means, the <u>remaining number of pies after Karen bought</u> will be:
10 more pies were sold during the sale and there were 19 pies remaining when the sale was over. So, the equation will be......
So, there were 60 pies before the bake sale started.