Formulas in Word Tables

I am trying to create a formula where if a number is higher than 20 then it is substracted from 20 then the difference is multipled by 80. If the number is lower than 20, then I need a 0 to be displayed. I know I am no explaining this well, let megive you an example: If cell A1 = 21 then insert 1 into cell B1 and then multiple that number by 80 and insert that total in C1. But if cell A1 = 19 then insert 0 into cell B1 and C1.

Can someone please help me.
Thanks,
Adele