C is the correct answer. A line is proportional if it is a straight line that passes through the origin, and while it does pass through the origin, it isn’t completely straight.
75% of something is 3/4 as 75/100 is simplified to 3/4. So 3/4 of 200 is 200÷4=50×3= C) 150
Note that f(x) as given is <em>not</em> invertible. By definition of inverse function,
which is a cubic polynomial in 
with three distinct roots, so we could have three possible inverses, each valid over a subset of the domain of f(x).
Choose one of these inverses by restricting the domain of f(x) accordingly. Since a polynomial is monotonic between its extrema, we can determine where f(x) has its critical/turning points, then split the real line at these points.
f'(x) = 3x² - 1 = 0 ⇒ x = ±1/√3
So, we have three subsets over which f(x) can be considered invertible.
• (-∞, -1/√3)
• (-1/√3, 1/√3)
• (1/√3, ∞)
By the inverse function theorem,
where f(a) = b.
Solve f(x) = 2 for x :
x³ - x + 2 = 2
x³ - x = 0
x (x² - 1) = 0
x (x - 1) (x + 1) = 0
x = 0 or x = 1 or x = -1
Then 
can be one of
• 1/f'(-1) = 1/2, if we restrict to (-∞, -1/√3);
• 1/f'(0) = -1, if we restrict to (-1/√3, 1/√3); or
• 1/f'(1) = 1/2, if we restrict to (1/√3, ∞)
You want to compare the square root of 55 using "mental math". Start off by choosing two perfect squares that you can think of that are close to 55.
If you don't know perfect squares then start with the number 2 and multiply it by itself. 2 times 2 equals 4, so 4 is a perfect square.
Take the number 3, multiply it by itself, and so on. Do this for all the numbers until you find two perfect squares that are close to 55.
The two perfect squares closest to 55 are the square roots of 49 and 64. Find the square root of these numbers.
√49 = 7
√64 = 8
Calculate how far 55 is from 49 and 64. 55 is 6 digits away from 49 and 9 digits away from 64.
This means the square root of 55 will be closer to the square root of 49; 7. Since we know that it will be closer to 7, you can put the less than sign for your answer.
√55 < 7.7
(The actual square root of 55 is ~7.4, so we were correct in determining the answer without using a calculator!)
