To find the total number of cans in the display, I need to add up all the terms of the arithmetic sequence. An arithmetic sequence is a sequence where each term is obtained by adding or subtracting a constant value from the previous term. In this case, the constant value is -3, since each layer has 3 cans less than the layer below it. I need to find out how many terms (or layers) there are in the sequence, so I can plug in n. I know that a_1 is 28, a_n is 1, and d is -3, so I can plug them in and get 1 = 28 + (n-1) (-3). Simplifying this equation, I get -27 = (n-1) (-3), or 9 = n-1, or n = 10. So there are 10 layers in the display, and 10 terms in the sequence. Now I can plug in n = 10, a_1 = 28, and a_n = 1 into the formula for S_n and get S_10 = 10/2 (28 + 1). Simplifying this, I get S_10 = 5 (29), or S_10 = 145. Therefore, the total number of cans in the display is 145. 
The number of cans in the layers of a display in a supermarket form an arithmetic sequence. The bottom layer has 28 cans; the next layer has 25 cans and so on until there is one can at the top of the display. How many cans are in the entire display?