My buddy was a general contractor. They have books of pre-printed calculators for common beam lengths. For instance, say you have a room 30 feet wide and you're putting a roof on it with a 30 degree pitch. The book will tell you exactly how long to cut the roof timbers so that they reach from the edge of the wall to the crest of the roof.

Said friend was at a site and someone had misplaced the book. He pulled out a calculator and did some basic trig to give them the lengths and told them to get back to work. He said they were looking at him like he'd just conjured a demon or something. "You can... just calculate that?" "How did you think they made the book?" "But how'd you learn to do that?" "In that math class you dropped in high school."