Grease and flour 9 x 13 x 2 pan, spread the 5 cups of rhubarb on the bottom of pan,cover with the dry strawberry jello, 1 cup sugar and 1/2 pound marshmellows. Mix the cake as directed and pour over the top of mixture.
Bake at 325* for at least 50 Min.
I serve from the pan and turn it over so the rhubarb is on top and top it with cool whip.
or if you want you can turn it out on another pan while still hot.