Grandma's Lemon Meringue Pie

Filling:

• 1 cup white sugar
• 2 tablespoons all-purpose flour
• 3 tablespoons cornstarch
• ¼ teaspoon salt
• 1 ½ cups water
• 2 lemons, juiced and zested
• 2 tablespoons butter
• 4 egg yolks, beaten
• 1 (9 inch) pie crust, baked

Meringue:

• 4 egg whites
• 1/2 cup white sugar