To find this solution for one set of stamps you can use the equation (x*0.45)+13.50 x being the amount of days they increase in value. You can do this with the 20 sets of stamps by multiplying 13.50 by 20 to get 270. So you get the equation (x*0.45)+270. Replace the x with the value 5, for the amount of days they give. so (5*0.45)+270 5*0.45=2.25. then add the 2.25 to the 270 and you get 272.25
