heat oven 350 degrees. in a 9x13 pan melt mararine in oven. sprinkle brown sugar evenly over melted margarine. arrange strawberries over brown sugar, then spinkle rhubarb over strawberries. press gently into brown sugar.
beat cake mix, water, oil, and eggs on low speed for 30 seconds. beat on med. speed for 2 min. pour batter over strawberries and rhubarb.
bake for about 50 min. or until toothpick insert comes out clean. immediatly run knife around edges. place a heatproof plate over pan, turn pan over. let set for 11 min. then remove pan.
cool 30 min. serve warm or cool. store loosely covered at room temperature.