Given:
The formula for total cost is

where, p is the price of item and s is the sales tax rate (as a percent).
You pay $14.77 for an item priced at $14.
To find:
The the tax rate.
Solution:
You pay $14.77 for an item priced at $14. So,
Total cost (T) = $14.77
Price of item (p) = $14
Putting T=14.77 and p=14 in given formula, we get



Multiply both sides by 100.

Divide both sides by 14.


Therefore, the tax rate is 5.5%.
F(f-1x) will be equal to x
take a simple example
f(x) = 2x
so x = f(x) / 2
f-1(x) = x/2
f(f-1x) = 2 ( x/2) = 2 * x/2 = x
15 is the answer because 8-5 is 3 and 3 and 4 is 12 so 12 plus 3 is 15