A proportion looks like this:

It means that the quantity a,b,c and d are in the same proportion: the ratio between a and b is the same ratio between c and d.
They are particularly useful in situation when you have an example given, and you want to extrapolate the relationship for another couple.
For example, you can think of the following problem: "Five apples cost 2 dollars. How much will 8 apples cost?"
You can build the proportion

And then solve it for x:

The answer is C. you can check the attached picture.
Answer:
m<b=107º
Step-by-step explanation:
180-41-32=107
D and B are congruent
Answer:
2x-10+x+x+10=180, or 4x=180
Step-by-step explanation:
So since the total is 180, you add them all up like an equation.
So,
2x-10+x+x+10
Combine like terms
4x=180
x=45
Then plug it back in to the equations.
x=45
(2x-10)=80
(x+10)=55