An easy, quick, and pretty dessert to make for family or guests. This makes a fitting patriotic dish for picnics.
Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Mix rhubarb, sugar, strawberry gelatin mix, and tapioca in a bowl. Spread the rhubarb mixture into the bottom of the prepared baking pan.
Beat white cake mix, water, vegetable oil, and eggs in a large mixing bowl until moistened; beat with an electric mixer on medium speed for 2 minutes. Pour the cake batter over the rhubarb mixture.
Bake in the preheated oven until a toothpick inserted into the middle of the cake layer comes out clean, about 35 minutes. Let cake stand for 5 to 10 minutes before inverting on a plate.