CSS calc() for inverse width isn't working - css

Question

I have an element that I want to increase in width as its parent decreases in width
The equation should look something like:
width:calc(150 + 500 / 100%);
But at least in Chrome it says the property is invalid whenever I try to divide by percent width.
Is this possible? (Alternatives to calc() are acceptable)
EDIT
I added spaces (didn't realize about that). Tried it with a variety of units, no luck yet.
Fiddle
<div style="width:100%;position:relative;">
<div style="width:calc(150px + (500 / 100%));position:absolute;top:0;left:0;">This one should get bigger as the page gets smaller</div>
</div>
Thought process:
Fixed width (150px) plus 500 divided by the current parent width.
So if the parent is 500px:
150 + 500/500 -> 150 + 1 = 151
Parent is 100px
150 + 500/100 -> 150 + 5 = 155
Parent is 20px
150 + 500/20 -> 150 + 100 = 250

Answer 1

Solution is so simple it's mind-boggling. Move the 500px into the first part and subtract the width.
width:calc(650px - 100%);
Gets wider as its parent gets narrower.
Updated fiddle

Answer 2

It seems like you've found your solution already, but I'll answer specifically why your original code wasn't working:
I'll start with the syntax for a product equation (when you divide) in calc():
<calc-product> = <calc-value> [ '*' <calc-value> | '/' <number> ]
The spec for the calc() property syntax is a bit more complicated than it sounds. When dividing in calc() as you're doing here, the right side must be a number. You cannot use "unit-ed" values:
Number values are denoted by <number>, and represent real numbers, possibly with a fractional component.
When written literally, a number is either an integer, or zero or more decimal digits followed by a dot (.) followed by one or more decimal digits and optionally an exponent composed of "e" or "E" and an integer. It corresponds to the production in the CSS Syntax Module. As with integers, the first character of a number may be immediately preceded by - or + to indicate the number’s sign.
That is to say, you cannot divide something by a percentage value like 100%.

Answer 3

calc() need spaces between the operators ( but just in + and -), but you are missing the units, which could be px, em,rem etc..., so would something like this:
width:calc(150px + (500px / 100%))
This is would be invalid because as explained by #TylerH, you can't divide by un-ed values(px, %, etc).
But if was possible, when you divide by 100% you are multiplying by 1, so basically you'll stay the same because 1 is the neutral value for multiplication, so this would be useless to do.
Hardly to know, because there isn't much to see in your question, and still invalid, but I'm guessing you are looking for something like this instead:
width:calc(150px + (100% / 500px))
Given your Edited question AFAIK you have to use JS to achieve this, unless you could provide a Fiddle.