At the end of a snow storm, Allison saw there was a lot of snow on her front lawn. The temperature increased and the snow began to melt at a steady rate. There was a depth of 20 inches of snow on the lawn when the storm ended and then it started melting at a rate of 1.25 inches per hour. Write an equation for S, S, in terms of t, t, representing the depth of snow on Allison's lawn, in inches, tt hours after the snow stopped falling.