-2, -5, -8. The pattern shows that the numbers decrease by 3.
20, 180 - 20, 180 + 20 and 360 - 20
i.e. 20°, 160°, 200° and 340°
It's a factor. This concept is widely used throughout algebra, and you'll probably bump into it through the end of high school and beyond.
A common use is expressing a term in <em>prime factorization</em>, or reducing a number to its most base parts- primes. For example:

Of course, a number like 13 which is already prime is made up of itself and 1. <em>Factors do not have to be primes.</em> 20 is also reducible through combinations of 1, 2, 4, 5, 10, and 20. Prime factorization is just a handy example.
Basically, factors multiply with each other to create other numbers, and numbers can be reduced down to their factors.
Answer:
$1.5
Step-by-step explanation:
12 x 1.75 + 6c = 30
21 + 6c = 30
6c = 30 - 21
6c = 9
c = 9/6
c = 1.5
$0.10 since you divide the price by the fluid ounce. (1.19/12=0.991...)