Multiply by 9: times 10, minus once
9 is 10 minus 1. Add a zero, take the number away once, done.
- Take the number, like 37.
- Multiply by 10 by adding a zero: 370.
- Subtract the original number once: 370 − 37 = 333.
Tip: The same idea works for 99: 99 × 14 = 1,400 − 14 = 1,386.
Try it now: Do 24 × 9 and 68 × 9 in your head. (216 and 612.)
Next crumbs
- 🧮 Mental mathMultiply any two-digit number by 11Split the digits apart and put their sum in the middle.
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