Problem 11) Start with set U = {1,2,3,4,5} and cross off 1,2,3 to be left with A' = {4,5}. We basically erase everything found in set A. You have the correct answer. Nice job.
Problem 12) Start with U = {1,2,3,4,5} and erase '5' to be left with {1,2,3,4}. So B' = {1,2,3,4}. You are correct here as well.
I can't help with the other problems since either letters, numbers or symbols are missing. Please make a new post and provide those updates.
A' (1,4)
B' (5,8)
C' (5,4)
D' (4,2)
When Area of rectangle is constant, then the length x is inversely proportional to the width y of the rectangle.

We know that the area of a rectangle is given by the product of the length and width of the rectangle.
Area = Length*Width
In this given problem,
The length of the rerectanglctangle is represented by x
and the width of the rectangle is represented by y
If the area of the rectangle is represented by A now.
So by the formula,
A = x*y
When A is constant then,
x = A/y

So from the above calculation we can conclude that about relation between length and width that,
"When Area of rectangle is constant, then the length x is inversely proportional to the width y of the rectangle."
Learn more about Area here -
brainly.com/question/25292087
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To Find :
How many gallons of a 80% antifreeze solution must be mixed with 100 gallons of 20% antifreeze to get a mixture that is 70% antifreeze.
Solution :
Let, x gallons of 80% antifreeze solution is required :
So,

Therefore, 500 gallons of 80% antifreeze solution is required.
12a)
5x + 45 + 3x - 25 = 180
8x + 20 = 180
8x = 160
x = 20
12b)
2x - 5 = 2(20) - 5 = 40 - 5 = 35
3x - 25 = 3(20) - 25 = 60 - 25 = 35
y = 180 - (35+35)
y = 180 - 70
y = 110