That will give you 20 0 14 on the stack (top to the right). After 20 ENTER 8 ENTER you have 20 8 8 on the stack (and the stack lifter disabled). - gets you 20 0

That will give you 20 0 14 on the stack (top to the right). After 20 ENTER 8 ENTER you have 20 8 8 on the stack (and the stack lifter disabled). - gets you 20 0; 7 ENTER gets you 20 0 7 7 (and the stack lifter disabled again); + gets you 20 0 14. If you want to compute 20 - 8 + 7, type 20 ENTER 8 - 7 + . As another comment points out, you only use ENTER to separate two numbers. Other ENTER:s are not optional, they are usually downright wrong, unless duplicating the number at the top of the stack is what you want. (Edit: as I am mentioning the stack lifter, I should probably say "bottom" rather than "top". The HP calculator stack was usually visualized as being above the display, with operations taking place at the display, i.e. at the bottom of the stack.)