Multiply any two-digit number by 11
Split the digits apart and put their sum in the middle.
- Take a two-digit number, like 36.
- Pull the digits apart: 3 _ 6.
- Add them, 3 + 6 = 9, and put the sum in the gap: 396.
- If the sum is 10 or more, carry the 1: 78 × 11 → 7, 15, 8 → 858.
Try it now: Do 54 × 11 and 87 × 11 in your head. (594 and 957.)
Next crumbs
- 🧮 Mental mathSquare numbers ending in 565² in two seconds: multiply the front by the next number up, then write 25.
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