First calculate after tax tire price
$390.60 / 4 = $97.65 after tax price per tire
take the $97.65 divided by 90
97.65 / 90 = 1.085
1.085 is the rate of tax he paid per tire
For any equation,

assume solution of a form, 
Which leads to,

Simplify to,

Then find solutions,

For non repeated real root y, we have a form of,

Following up,
For two non repeated complex roots
where,

and,
the general solution has a form of,

Or in this case,

Now we just refine and get,

Hope this helps.
r3t40
It's sometimes, because let's say I have 4/5 right? So when in a percent it's 80%. I suppose you could turn it into a a decimal of 80/100 but no not always. 3295729357/1000000000000. That doesn't have a denominator of 100, so yeah.
Answer:
2√58
Step-by-step explanation:
I believe the answer is 2√58.
The answer would be 5*C or 5C or 5(C) they all mean the same exact thing but if Kevin has 5 times as much no matter what billy has you’ll have to multiply that by 5 to see how much Kevin’s has so if billy has c then you multiply that by 5