Answer:
8
Explanation:
Let's call the center of the middle circle J, and the center of the third circle K.
Draw a line from J to the line AD such that the line is perpendicular to AD.
This forms a right triangle that is similar to the triangle ΔADK.
Writing and solving a proportion:
x / DK = AJ / AK
x / 5 = 15 / 25
x = 3
Now, draw lines from J to B and C. We now form two congruent right triangles, where the hypotenuse is 5 and the shared leg is 3.
That means these are both 3-4-5 triangles.
Therefore, BC = 4 + 4 = 8.