If you actually draw the diagram described in the text (Use something like Microsoft Paint if you're using Microsoft Windows), you'll quickly realize that the detour is a 30/60/90 right triangle. The 90 and 30 degree values are given in the text. And since all triangles have a total of 180 degrees, the 3rd angle is 180-90-30 = 60.
Now for a 30/60/90 right triangle, if you consider the hypotenuse to be of length 1, the short leg will be of length 0.5 and the long leg will be of length sqrt(3)/2. Those are values that you should have memorized. And if you haven't memorized them, you can easily derive them from realizing that if you take the triangle and copy it, then rotate the copy around the long leg, you'll get an equilateral triangle with one of the 60 degree angles bisected. So you'll quickly realize that the hypotenuse has a length of 1, the short leg will be half of that, and the long leg you can calculate using the Pythagorean theorem. And for this problem, the short leg is of length 4. So:
4/0.5 = X/(sqrt(3)/2)
8 = X/(sqrt(3)/2)
(sqrt(3)/2)*8 = X
sqrt(3)*4 = X
1.732050808*4 = X
6.92820323 = X
And after rounding to 1 decimal place, you get 6.9 miles.
