(1916) Dodd Mead & Co. pp270-271*"Ratio and Proportion" "Fundamentals of practical mathematics", George Wentworth, David Eugene Smith, Herbert Druery Harper (1922) Ginn and Co. pp.
(1916) Dodd Mead & Co. pp270-271* «Ratio and Proportion» "Fundamentals of practical mathematics", George Wentworth, David Eugene Smith, Herbert Druery Harper (1922) Ginn and Co. pp.