Answer: We are missing some data here, but it appears to be a right triangle question. Assuming that triangle ABC is a right triangle, the measure of angle B would be about 63.26 degrees.
If you draw a right triangle ABC with the given information, you will see that we have an adjacent side and hypotenuse with regard to angle B.
Therefore, we can write and solve the following cosine ratio.
cos(B) = 9/20
B = 63.26 degrees