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Angle A subtends to arc BC, which is equal to 180. Therefore angle A is 180/2 = 90. Knowing that it is a 45-45-90 triangle, can you figure out the area of the triangle? After that, just subtract it from the area of the semi-circle.

By Thales' Theorem: angle BAC = 90 degrees(it subtends an arc of 180 degrees)

Because we know that, we know that triangle BAC is a right triangle with angle measurements of 45, 45, 90

Next, realize that the hypotenuse of said triangle is the diameter BC, which has length 16( which is 8 *2).

Now that we have our hypotenuse, realize that the ratio of a 45-45-90 triangle is 

x, x, $x\sqrt{2}$

That means that our leg of our right triangle is equal to:

$16\over{\sqrt{2}}$= $16\sqrt{2}\over2$ by rationalizing the denominator(multiplying it by $\sqrt{2}\over\sqrt{2}$)

This equals 

$8\sqrt{2}$

Triangle area formula is 1/2 * b * h

Since the base and height are the same in this scenario, we have that the area of triangle BAC is:

$(8\sqrt{2})^2/2 = 64*2/2 = 64$

The area of the entire circle is 64pi, so half of it is 32pi. Our area of the shaded region is then:

$32\pi-64$

The probability is then this part divided by the entire area of the dart board, which is 64pi, so our answer is:

$32\pi-64\over64\pi$

Area of circle = pi r^2    =    64 pi   

      subtract 1/2 (unshaded area) = 32 pi

         subtract area of triangle  1/2 b h = 1/2 16 8 = 64

(32 pi - 64) / (64 pi)