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Mariana Parra Rosado attends a Mayan mathematics class at Escuela Modelo primary school in Merida, Yucatan, Mexico on December 19, 2017. Maya mathematics constituted the most sophisticated mathematical system ever developed in the Americas. The Maya counting system required only three symbols to form all their numbers: a dot representing one, a bar representing five, and a shell representing 20 or zero, depending on its placement. The Maya used a base 20 (vigesimal) numerical system, unlike our current base 10. (Photo by Bénédicte Desrus)