A percentage can just be written as a decimal where 100% = 1.00
15% = 0.15
75.5 * 0.15 = 11.325
You just multiply.
Let r be a radius of a given circle and α be an angle, that corresponds to a sector.
The circle area is

and denote the sector area as

.
Then

(the ratio between area is the same as the ratio between coresponding angles).

.
Yes, 104.12 is greater than 104.002
because 104.12 rounds up to 104.1
and 104.002 rounds up to 104.0
Here you go. Let me know if you have questions