Black plus black is black
Imagine the days when computers were human and usually semi-literate. Mathematicians developed approximation algorithms that were turned over to these people to sum up (subtraction disallowed). Now the story is told from the computed return of Halley's comet to nuclear bomb yield computations in David Alan Grier's book, When Computers Were Human. A review is in slashdot.org and the book can be purchased at Barnes and Noble or Amazon.