Pentagonal and Hexagonal Numbers/Mistake

Source Work

 * The Dictionary
 * $40,755$
 * $40,755$

Mistake

 * The first number, after $1$ and $15$, to be simultaneously pentagonal and hexagonal and therefore, also, triangular.

$15$ is not a pentagonal number as such.

It is, however, a second pentagonal number, and so is classified as a generalized pentagonal number.

If this is what meant, then he should have stated it.