How do I determine "adjusted cost base" if I have cost or book value (box 20), proceeds of disposition (box 21) and quantity of securities (box 16) (for mutual funds)?

I'm assuming it's going to be a calculation of some sort. Can you give me a clear example of I give you the fake values for box 20 = 11, box 21 = 10, and box 16 = 50.85? I have taken a loss in the sale. I'm a bit frustrated that TurboTax doesn't match the layout of the T5008 and leaves me guessing.