Answer:
Since we know the distance from Susan and Tammy is 20 miles and the ufo is between them, then the ufo is 10 miles from Susan.
The angle of elevation is 15 and we can use the cosine ratio- adjacent/hypotenuse(which is the answer we're looking for)
Cos15 = 10/x
cos15(x) = 10/x(x)
cos15(x)/cos15 = 10/cos15
x = 10/0.96592583
x= 10.35 (which is your answer)