You have a right triangle.
For the angle you are looking, 30 is the adjacent leg.
31 is the hypotenuse.
The trig ratio that relates the adjacent leg and the hypotenuse is the cosine.
Let's call the unknown angle measure x.
cos x = adj/hyp
cos x = 30/31
cos x = 0.96774
To find x, you need the inverse cosine of 0.966774.
x = 14.592
Answer: 15 deg.