For the sake of conversation we will assume that you are going to use 4x8 sheets. We will also assume that the first dimension you have given is the width and the second the height. For the first wall you will need 11 sheets (4x8 sheets). 11 sheets will cover an area 20'x16'. For the second wall you will need 5 4x8 sheets to cover the 11'x10' wall leaving you enough to finish the last foot of the first wall. Finally, you will need one sheet to finish the last wall. If you use 4'x10' sheets then you will need 12 sheets instead (one sheet less). Keep in mind that our takeoff assumes that your stud walls are laid out on perfect 16" centers. Chances are you will need one or two more sheets than we recommend if the stud layout is not perfect.