What' wrong with this?

Let S = the amount of initial grass
g = the growth rate of grass
c = the eating rate of cow
D = days for 1 cow
then
1). For 3 cows, 3*3*c = s + 3g
2). For 2 cows, 2*6*c = s + 6g
3). For 1 cow, 1*D*c = s + Dg
There is no solution for this equation system