The answer is 6$ because 50% of 40 would be 20 and 50% of 20 is 10 but when you all on the tax which would be around a dollar when you take 50% of 10 which is 5 and add on the tax which is a dollar you then get 6$ in total.
The cotangent function is defined as the ratio between cosine and sine of a given angle, i.e.

Since you can't have zero at the denominator, the cotangent function is not defined when the sine is zero.
Let's look at your option:
, so the cotangent is defined here
, so the cotangent is not defined here
, so the cotangent is defined here
, so the cotangent is defined here
She should use 0.74 gallons you have to take 20 and timed it by 3.7 % and thats how you get to see whats left that she need.
3y³-y²+8y-1+5/y-4. is that unless you want the answer smaller