Underhill, R. G. (1977). Teaching elementary school mathematics (2nd ed.). Merrill.
Cita Chicago (17th ed.)Underhill, Robert G. Teaching Elementary School Mathematics. 2nd ed. Columbus: Merrill, 1977.
Cita MLA (9th ed.)Underhill, Robert G. Teaching Elementary School Mathematics. 2nd ed. Merrill, 1977.
Atenció: Aquestes cites poden no estar 100% correctes.