Compute 993 × 27 in your head

Question

Solve in your head, no paper. What is 993 	imes 27? Aim: under 12 seconds.

Accepted Answer

Round 993 up to 1000, then subtract seven times the other factor: 993 	imes 27 = 1000 	imes 27 - 7 	imes 27 = 27000 - 189 = 26811. The correction is easy: 7 	imes 27 = 189, and 27000 - 189 = 27000 - 200 + 11 = 26811. Takeaway. The round-and-correct anchor scales to 1000 just as well as 100: a factor within single digits of a round power of ten turns a nasty multiply into "round times other, minus small correction." Pick the anchor closest to the awkward factor.

Compute 993 × 27 in your head

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