HP HP-28S - Two-Number Functions; Addition and Subtraction; Multiplication and Division

Two-Number Functions
Functions that act on two numbers-such as addition-are called
two-number functions. All act on the numbers in levels 1 and 2.
When you're keying in both arguments to the function, as when you
divided 2 by 3 on page 36, you must press 1 ENTER 1 to separate the two
arguments.
When one or both arguments are already on the stack
from previous calculations, you don't need to press 1 ENTER I.
Addition and Subtraction
Calculate 36 + 17.
36 1 ENTER I
17 ~
The result is 53.
3: -4.2E-12
2: .125
1: 53
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For addition the order of the numbers doesn't matter. However, the
order is important for subtraction. Next calculate 91 - 27.
91 I ENTER I 3: .125
2: 53
27 G 1: 64
The result is 64.
Multiplication and Division
Calculate 13 x 6.
131ENTERI
60
The result is 78.
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3: 53
2: 64
1: 78
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2: Doing Arithmetic 41