As stated in the text, the central pole forms a right angle with the floor. The side of the tent corresponds to the hypotenuse of the right triangle formed with the pole and the floor.
The cosine of an angle is the ratio between the side adjacent to it and the hypotenuse of the triangle. In this case, the cosine of the missing angle is equal to 20ft divided by 26ft, which is approximately 0.77.
Applying the arccos function, we can find the angle whose cosine is equal to 20/26:
Then, the angle that the tent pole makes with the sides of the tent is approximately 39.7º.