Brain Teasers & Mental Math
Fast trick: multiply by 11 in your head — what are 45 × 11 and 78 × 11?
Model answer
495 and 858.
Trick for two-digit × 11: spread the digits apart and insert their sum in the middle. 45 × 11 → 4 _(4+5)_ 5 → 495.
With a carry: 78 × 11 → 7 _(7+8)_ 8 → the…
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