You have stated that the tax rate is 8.875% and the customary tip is 18%.
To set up fomulas to figure each component, we could do the following:
b is the amount billed
x t is the tax
p is the tip
t is the total amount due
With these definitions, we can figure out each component and add the totals together. Note, the percentage is divided by 100 to get is decimal value.
x = b * 0.08875
p = b * 0.18
t = b + x + p
So, with a bill of $28,
x = 28 * 0.08875 = 2.485 = 2.49 (rounded to nearest penny)
p = 28 * 0.18 = 5.04
t = 28 + 2.49 + 5.04 = $35.53
We might notice that each component is calculated from the amount due (b), which would allow us to simplify the expression by combining the like terms. Since "t = b+x+p" is the bill plus tax and tip, we could setup the formula like this:
t = b + (b * 0.08875) + (b * 0.18)
Then we can simplify it:
t = b + 0.08875b + 0.18b
t = 1.26875b
Now plugging the $28 of the amount due on the bill, we can calculate the total amount due including the tax and tip.
t = 1.26875 * 28 = 35.525 = $35.53 (round up to the next penny)
Either way we will arrive at the same answer.
From the practical application side, we might notice that the tip (18%) is just over twice (2x) the tax (8.875%). Since the amount of the tax is listed on the bill, a simple solution is to double the tax and then round up. So if the bill before tax is
$28 and the tax added is $2.49 (bringing the bill to $30.49 before tip), then doubling the amount of the tax gets us $4.98. We can round that to $5. This would make the bill $35.49, just $0.04 under the $35.53 of an exact calculation. At that point we might
round up to $35.50 or even $36, which means the server gets a little extra in the tip.