The &quot;remove 1&quot; trick is sort of how I learned it, except instead of memorizing &quot;2 goes with 7&quot; I learned that the 2 digits should add back up to 9:<p><pre><code>  9*3 = 27 (3-1 = 2,7 = 9-2)
  9*4 = 36 (4-1 = 3,6 = 9-3)
  ...
  9*9 = 81 (9-1 = 8,1 = 9-8)
</code></pre>
Fun stuff :)