Finding distances with a compass

i just found this compass/clinometer, the suunto tandem. im reading the manual and it has this on finding distances using the compass:

You measure a hill at 0*(magnetic north). The angle between the curve of the road and the hill is 64*. The angle between the curve of the road and the oil derrick is 15*. A line is drawn at a 90* angle to the 64* bearing line from the curve of the road toward the oil derrick bearing line. The distance, as measured on the chart on the compass is 1 mile. Then your position is cot 15* x 1 mile(1.6km) = the distance along the corrected bearing line of 64*.

uhh, what? i just finished trig class so i know the basics of this stuff but im confused as to how you can find a distance without knowing any of the other sides. Because, i can measure a 15* angle from here to my computer, and its not 3.7 miles.

heres a link to the manual (page 10)

[url]http://www.suunto.com/media/suunto/manuals/multilangual/tandem_users_guide_36b13.pdf[/url]