A fun property I learned that helps me remember the 9s: The digits of every multiple of 9, when totaled, eventually add up to nine.<p>For 9 times x, for 2-10 at least, this can be reverse engineered: The 10s digit of the product will be x-1, and then the 1s digit will be whatever number remains that&#x27;s needed for the digits to add up to 9.<p>So 9 times 7: 7-1 = 6, 6+3 = 9, therefore 9*7 = 63.<p>There may be a similar trick for bigger numbers, but I rarely need to do them quickly so I&#x27;ve never found one.