Half of 40 = 0.5 x 40 = 20 .
One fourth of 20 is smaller than 20.
You don't even have to calculate it.
There's probably a mistake in the problem; as written the answer is none of the above.
We have slope from the first two points of (4-2)/(0-4) = -1/2 and a y intercept from the second point of 4 so
f(x) = (-1/2) x + 4
maybe that's the third choice; forgot the fraction bar.

looking at the picture above... you do have the radius, is just in the truncated part of the picture... so you have "h" and "r", so just plug them in
12/51 x 100 = 23.53%
Answer: 23.53%
The solution is x=16. hope it helps