A general guide for the keys of calculators

Here there are some videos for the general use of key in modern calculators.
But some of these keys are also in older calculators.

1: position of the fingers, basic operations, comma, reset, total, CE

2: Multiplication

3: Division

4: Gross Margin (GM)

5 Print and ACC Key

6 R/O Key (rounding)

7 Selection of the comma

8 Calculation with date, hours, days