To switch the HP 20b calculator in Algebraic mode press [Shift][Mode]. Press [Up Arrow] or [Down Arrow] repeatedly to scroll through the menu till you see " Algebraic" or " RPN" item is displayed, press[INPUT] key to cycle through till you see “Chain”. To exit the Mode menu, press [ON/CE] key once.
Calculations in Chain mode are interpreted in the order in which they are entered. For example, enter the following numbers and operations as written from left to right: 1+2*3. If you press an operator key, +-*/, after =, the calculation is continued using the currently displayed value. So in Chain mode, 1+2*3 = 9.
In Chain mode, if you wish to override the left to right order of entry, use parentheses () to prioritize operations. For example, to calculate 1+ (2 x 3), you may enter the problem as written from left to right, with parentheses to prioritize the multiplication operation. Press [=] the answer of 7.0 will be displayed.