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?
"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 :$$17\times\frac{1}{100}x = \frac{17}{100}x = 0.17x$$
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.