Does 17% have to be equal to 0.17? I have a friend who believes that 17% doesn't have to be equal to 0.17.  Even though he says that 17% is equal to 0.17 on its own, he says that 17% at any other time is not equal to 0.17, referring to the argument that $17\%x \neq 0.17$. No matter how I try to explain it to him, he won't believe me when I say that 17% is always equal to 0.17, no matter what. Does anyone have a good explanation for this?
 A: I think your friend is more on the right track than you are. What you're confused about is how to treat the phrase 17% in language, not in mathematics.
He understands that $17\% = 0.17$, in the sense of the term where $17\%$ is an isolated figure. You're trying to convince him that this is the only valid usage of the term $17\%$.
But consider a sentence like this:

On a successful sale, you'll earn anywhere from \$12,000 to \$18,000, and the real estate agency will take 17%.

In this sentence, interpreting $17\%$ as $0.17$ makes absolutely no sense. It's obviously $17\%$ of the \$12,000 to \$18,000 you earn from a successful deal, which is not $0.17$ at all, but rather around two to three thousand dollars.
And something like this:

This week, viewership of our front page went up by 17%.

Viewership can't really go up by $0.17$ (because you can't get $0.17$ visitors to a site). It's referring to a percentage relative to the past week. So if you had 20,000 visitors to your site, you'd now have 23,400 visitors, which represents an increase of about 3,400 visitors – again, nothing to do with $0.17$.

Basically, what's confusing you is how context affects the use of the percentage term. Yes, when you say $17\%$, you're always calculating something multiplied by $0.17$. But this is very different from saying that $17\%$ is equal to $0.17$ in that case.
A: Your friend agrees that

17% is equal to 0.17 on its own

and hopefully he would agree that
50% of something = halve of something = 0.5 of something

and similarly, 
17% of something = 17/100ths of something = 0.17 of something

Therefore in both ways of referring to 17% (on their own and in relation to some other value) it seems to be fully equivalent to just saying 0.17.
A: "17 per cent" on its own is $\frac{17}{100} = 0.17$. That's what it means in English language and I'm pretty sure it's the same in most languages.
However $17\%$ of something, say $x$, will be $\frac{17}{100}x = 0.17x$ which of course isn't $0.17$ except for the special case $x = 1$ but that's not very interesting.
If this still doesn't convince you friend, you could take an example :
Say we have an object with a certain price $x$. Then $1\%$ of the price is like $1$ hundredth of the price which is :$$\frac{x}{100} = \frac{1}{100}x = 0.01x$$
$17\%$ of the price of the object is $17$ times greater than $1\%$ of the price therefore it is :$$17\times\frac{1}{100}x = \frac{17}{100}x = 0.17x$$
A: At least in my native language (German), you can't have "17%" on its own. You always have to refer (at least implicitly) to some quantity that the 17% are part of. So, at least in German, 17% is totally meaningless on its own - and it's not taught in school that 17% = 0.17 or 17% = 17/100.
17% is not recognized as a number but as a function (percent(17,x) = x/100*17) like for example we have "das Vierfache von" = "the quadruple of". (quadruple(x) = 4*x) I'm sure that also in English it does not make sense to have "a quadruple" on its own. Otherwise, would you say: A quadruple is 4?
Vice versa, in German, it's not even possible to say: "0.17 of something". The terms are not interchangeable from a linguistic point of view.
A: The term percent comes from the Latin per centum, or per hundred. 17 per 100 is 0.17, so 17 percent is most definitely 0.17
A: This question almost seems like it should be on English SE rather than Math. There is a failure to understand the English language more than there is a failure to understand the math; it seems everyone agrees on the numbers, it’s the words that are giving trouble.
"17% of something" means, in the English language, "17% multiplied by something," so yes, you can still replace 17% by 0.17: the statement just becomes "0.17 multiplied by something" and is still completely true.
A: I'm going with him on this one. We've come to accept that 17% = .17 because that's how it's interpreted in the context of math, but 17% and .17 are not the same thing semantically.
0.17 is simply a number. 17% is a function. Without another parameter (number you're calculating a percentage of), 17% is only meaningful in a relative sense.
Think of it this way:
If I go outside, I can jog for 0.17 miles (probably pretty accurate, too). I can't, on the other hand, jog for 17% miles. (I can jog for 17% of a mile, but again that's using 17% as a function.)
A: Ask him to calculate what 17% of some random high value is. Let him use a calculator. See what he presses.... I hope for him that he will enter your random value and multiply it by 0.17 to get to the answer :)
A: If your friend is not willing to accept that 17% of x is always 0.17 x then a simple way to make him believe will be to ask him to prove it otherwise. If he fails to prove his theory mathematically, its invalid. You cannot deny proofs in mathematics without demonstrating their invalidity mathematically. Ask him if its not always 0.17 of something then you'd like to see what it is, backed with mathematical reasoning.
A: The core issue here is not an issue of mathematics, but an issue of language. 17% is obviously only ever used to state a proportion of some whole and always has an explicit or implicit "of". While .17 can only really represent a portion of some whole unit (frequently the integer "1") we don't think about it in the same way.
Depending on your perspective the statement 17% = .17 is either always true or always false, but it's silly to say that sometimes it's true and other times false.
If you say that 17% is a number then the statement is absolutely true. If you say that 17% is not a number then it's impossible to ever say that "17%" itself is directly equal to any number.
A: "Per cent" means "per 100" because "cent" is the Latin root for "hundred". So 17% means 17 per 100, or $17/100 = 0.17$.
Even if you had 34 items out of 200 (two hundred), or 51 per 300 (three hundred), that's still $34/200 = 0.17$ or $51/300 = 0.17$. It all simplifies to a base of 100. As long as the base unit is 100 for dividing your value, it will always equate to a decimal out of 1.
If there was something called "per-dec" (per 10), or "per-milli" (per 1000), then it would vary based on that. But as far as I know, those are never used.
A: $17 \% \text{ of } x$ is the same as saying $.17x$. The actual value of $17\%$ depends upon the value of $x$. If $x=50$, $17\%=8.5$.
A: A percent always represents a fraction of a whole. what that whole is is undefined until you provide it. Just like hertz: hertz is a unit of measurement that means 1/seconds or per second, but not what is happening per second. Or like verbs in a sentence; by themselves they only define themselves, but within a sentence they can represent a cohesive communication. For future reference, this concept is the definition of a function. 
A: While 17% is mathematically .17 of whatever item you have, remember that we don't always work with real numbers. Sometimes we are limited to discrete values, like integers, and so there are times when your friend may be right.
As an example, imagine you have a formula for dividing a certain quantity of items among several people, such that one of those people gets 17%. Now say you have 100 items. Of course that person gets exactly 17, or .17 of the total. But now lets say that instead of 100 items to distribute you have 101 items. You can't break the items apart, but the formula still says this person should get 17%. So what happens? He still gets 17 of them. However, in this case, that 17% did not work out to exactly .17 of the total. Instead, a quick check of the calculator shows the result comes to .16831683168.
