First, you will notice that the page contains certain fields into which you are expected to type in values.
Therefore, never press the Return or Enter key. You will move from field to field by pressing the Tab key, and you will initiate calculations by clicking on a button that usually has a name like Compute, or Calculate, or something similar. (In some of my web pages, a calculation will be performed automatically when you tab out of a field.)
I've found that it's often safest not to try to make alterations to part of an existing field (like selecting a single digit and changing it), but rather to re-type the entire field.
So, as incredible as it may seem, many browsers will interpret the number 023 typed into a cell as having a numerical value of nineteen ! ! ! (That's because 2*8 + 3*1 equals 19.) The browser will give you no warning of this crazy behavior, only wrong results. As if this were not insane enough, those same browsers will correctly interpret the number 029 typed into a cell as having the numerical value of 29. (This is because the octal numbering system does not have the digit "9", therefore the browser interprets 029 as a decimal number.)
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