I'm a mathematician. When I was in grade school they told us to memorize up to twelve. But I figured I could skip the evens, because doubling is easy, so I would just memorize the odds. One is trivial, three and five are simple, as is eleven. There is an easy algorithm for mine. So I memorized seven and called it done.
When I think of the thousands of multiplications I've done since then, it's clear I was a dope and my teachers were perfectly right.
I noticed that there's really only a 4x4 square in the multiplication tables (6-9) that aren't obvious, and since the table itself is mirrored along the diagonal, there are only really 9 entries you truly need to memorize: 6x6, 6x7, 6x8, 6x9, 7x7, 7x8, 7x9, 8x8 and 8x9.
Suppose now somebody threw in that expression seriously. Would not that be an admonition to learn the tables and learn each element properly (do, check, memorize; do, check, memorize, check, check, check)?
When I think of the thousands of multiplications I've done since then, it's clear I was a dope and my teachers were perfectly right.