At the end of a snow storm, Amelia saw there was a lot of snow on her front lawn. The temperature increased and the snow began to melt at a steady rate. After the storm, the snow started melting at a rate of 1.5 inches per hour and it is known that 4 hours after the storm ended, the depth of snow was down to 12 inches. Write an equation for S, S, in terms of t, t, representing the depth of snow on Amelia's lawn, in inches, tt hours after the snow stopped falling.