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HP12C SOYD function:

HP12C SOYD function

I use the soyd function to calculate the percentage variation between two numbers.

$1200 -> $1700

Clearly, we have an increase of 41,67%.

But if I type $1700 first and then $1200.

$1700 -> $1200

The HP12C returns -29,41%.

Why is that?

I used to think any variation would result the same percentage, I don't understand where -29,41% comes from.

Thanks in advance.

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    Why would you expect it to yield the same result? 1700/1200-1 = +41.67%, conversely 1200/1700-1 = -29.41%
    – 0xFEE1DEAD
    Jun 15 at 15:45
  • The general formula for percent increase or decrease is (new-old)/old, not sure why you think it would be the same number in both directions.
    – eps
    Jun 15 at 16:12
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Percent changes are always relative to the starting value.

$1200 is $500 smaller than $1700. $500 is 29.41% of $1700.

Would you expect a change from $2400 to $1200 to be a "100% drop"?

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  • Although it sounds very obvious I couldn't figure it out logically, but thanks very much. Jun 15 at 17:06
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When you go from $1,200 to $1,700 the $500 gain is (500/1200) or a 41.67% gain.

When you go from $1,700 to $1,200 the $500 loss is (500/1700) or a 29.41% loss

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