Rhubarb and Strawberry Dump Cake

Ingredients (7)

• + 4 cups chopped rhubarb add to pantry
• + 2 cups sliced strawberries add to pantry
• + 1 (3 ounce) package strawberry-flavored gelatin mix (such as Jell-O®) add to pantry
• + ¼ cup white sugar, or more to taste add to pantry
• ✓ 1 (15.25 ounce) package yellow cake mix
• + 1 cup warm water add to pantry
• + ¼ cup butter add to pantry