For an object with a uniform constant density, the total has of the object is equal to Area times density. When the density is not constant to approximate the mass we need to divide the region into small pieces in such a way that the density is approximately constant on each piece and add the contributions of the pieces. we have a rectangular metal sheet that is forming a wall with length along the base of $32$ feet and height of $20$ feet. the density of the metal sheet depends on the height form the base given by y $=(40-$y$),$ when you are y feet above the base. Write a Riemann sum that approximates the mass of the sheet. Set up a definite integral that will calculate the mass of the sheet. Use the fundamental theorem of calculus to find the mass of the sheet $.$