Rule of Three
The Rule of Three (or Rule of Proportion) was the term for the mathematical formula used to find a fourth, unknown number from three given numbers, when the first is in the same proportion to the second as the third is to the unknown fourth. In the eighteenth century, masters of apprentices were sometimes required to teach them to write and decipher to the rule of three. Among the apprentices to whom this applied were tailors, house carpenters, cordwainers, and others; girls who were apprenticed to learn to spin, weave, or other domestic arts were not required to be so taught. In their last wills , fathers sometimes inserted the stipulation that sons be taught arithmetic to this point . In 1816 North Carolina Quakers  opened a school where black males could be taught to read and decipher to the rule of three but females would only be taught to write.
John Hope Franklin, "Slaves Virtually Free in North Carolina," Journal of Negro History 28 (July 1943).
Rule of Three, North Carolina schools and academies, 1790-1840, a documentary history, NCDCR Digital Collections: http://digital.ncdcr.gov/u?/p249901coll22,335114 
North Carolina journal of education, NCDCR Digital Collections: http://digital.ncdcr.gov/cdm4/document.php?CISOROOT=/p249901coll37&CISOPTR=14360&REC=4 
1 January 2006 | Powell, William S.