Calculate MUmax and Yx/s Given Growth Data

How do you calculate MUmax (hr^-1) and Yx/s (g DCW/g Substrate)  give the following information:

Time (hrs), Cell Mass (g DCW/L), Substrate (g/L)?
0,            .5,                 20
1,             1,                 19
2,             2,                 17
3,             4,                 13
4,             8,                 5