## The percent button on Casio calculators

For the discussion of math. Duh.

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Tirian
Posts: 1891
Joined: Fri Feb 15, 2008 6:03 pm UTC

### The percent button on Casio calculators

I don't know if this works better in the math section or the logic puzzle section, but it's something that I've always wondered about and I don't know if it's more trivia or deducable.

Anyways, I was teaching a GED math class and explaining why I'm not a big fan of using the percent key on the Casio fx-260. In a nutshell, it works fine for multiplying or dividing by percentages if the percentage is the second operand, but it doesn't work in other intuitive ways that I would expect. For instance, if I bought a \$36 item and I wanted to know how much it would cost when you add in 8% tax, my initial expectation is that the buttons 3 6 + 8 % would get me there. But it doesn't -- the result of that operation is 550. Now, I'm not asking why Casio isn't meeting my sensible intuition or how to really solve that problem with a calculator, but I'm burning with curiosity over what mathematical atrocities the calculator is doing to that unsuspecting 36 and 8 to turn it into a 550. Any insights?

Sagekilla
Posts: 382
Joined: Fri Aug 21, 2009 1:02 am UTC
Location: Long Island, NY

### Re: The percent button on Casio calculators

Maybe because the % operator acts in much the same way as a * or / operator?

Clearly if you did something like: 8 * (3 + 6)
This is not the same thing as: 8 * 3 + 6

That is, given some expression of the form A B % or B A %, the results are different.
In other words, the operator doesn't commute unless you put the parenthesis in by hand.
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Dopefish
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### Re: The percent button on Casio calculators

(36+8)/8=5.5=550%, that is, the percent increase when you add 36 to 8 is 550 percent.

I don't like percent buttons because I'm perfectly happy to multiply by 1.08 to find out what an 8% increase in the number is, and I don't need to worry about witchcraft like the above happening instead.

Tirian
Posts: 1891
Joined: Fri Feb 15, 2008 6:03 pm UTC

### Re: The percent button on Casio calculators

You win an internet for that! Well, maybe a slightly used internet. When you add 36 to 8 you wind up with 550% of what you originally had, which is a 450% increase. Still, wow, that's both weird and undocumented, so grats and thanks!

BTW, I totally agree. It's a healthy conversation to have with GED students that even when you do know how to use the calculator you're stronger yet when you can do some of the work in your head. Also, having just checked the user's guide, they expect that we would think to do that as 36 * 8% + or 36 * 8% - if we wanted to know the sale price of a \$36 item that was discounted 8%. Color me unimpressed.

ycc1988
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Joined: Fri Jun 22, 2012 5:13 am UTC

### Re: The percent button on Casio calculators

I've always found the % button confusing and useless. Does anyone know the historical reason of why it's there in the first place?

gorcee
Posts: 1501
Joined: Sun Jul 13, 2008 3:14 am UTC

### Re: The percent button on Casio calculators

ycc1988 wrote:I've always found the % button confusing and useless. Does anyone know the historical reason of why it's there in the first place?

Accounting.

Seriously, if you ever sit down with an older CPA or banker, and watch them punch things into a calculator, it's a.) astounding to watch and b.) completely, utterly frustrating.

Tirian
Posts: 1891
Joined: Fri Feb 15, 2008 6:03 pm UTC

### Re: The percent button on Casio calculators

That. Plus, you need to remember that those ten key machines they use have paper tapes to "show their work". 8% and .08 are two representations of the same number, but I'm sure that reading comprehension would be compromised if you used them interchangeably when showing a paper trail for a weekly expense report.