Es un ejercicio y herramienta para practicar la notación desarrollada en los niños. En realidad son papeletas hechas acordeón (ver imagen). O gráficas con espacios en blanco.
Aquí la explicación en inglés:
Number Expanders are useful tools because they offer a hands-on way of manipulating the symbolic representation (numeral) of a number. They make a bridge from physical to symbolic models for number. Opening and closing the number expander act as a reminder of the actions with materials (but not as a replacement for this). Because they do not physically model the size of the number, they can be made with any number of place value columns and so can represent very large or small numbers at more advanced levels.