Answer:
B. 10
Explanation:
This pair of triangles follows a scale factor. A scale factor is the factor that can get you an exact copy of a triangle, but with bigger or smaller side lengths. In this case, the scale factor is 2 (doing 12/6 on the left side is 2), and dividing that by 20 to get 10. Because we want the smaller triangle, we divide, not multiply. (you could also multiply by 1/2)