How do you multiply by 9 or 99 quickly?

Multiply by the round number above (10 or 100) and subtract the original.

Because 9 = 10 − 1 and 99 = 100 − 1:

• 47 × 9 = 470 − 47 = 423.
• 47 × 99 = 4700 − 47 = 4653.

Check: 47 × 9 = 423 ✓, 47 × 99 = 4653 ✓. This generalises: ×n9…9 is "shift and subtract." Far easier than the "finger trick," and it works for any size.