Compute 217 × 34 in your head

Question

Solve in your head, no paper. What is 217 	imes 34? Aim: under 20 seconds.

Accepted Answer

Split the awkward factor at the round boundary: 217 = 200 + 17. - 200 	imes 34 = 6800, free. - 17 	imes 34: split again, 17 	imes 30 = 510, 17 	imes 4 = 68, total 578. (Or notice 17 	imes 34 = 34^2 / 2 = 1156/2 = 578.) 6800 + 578 = 7378. Say the running total, not the parts: "sixty-eight hundred… seventy-three seventy-eight." Takeaway. Big-times-small splits: peel off the largest round chunk first so the leftover multiplication is 2-digit × 2-digit at worst. Holding one running total in words ("seventy-three seventy-eight") uses less working memory than holding two partial products and a…

Compute 217 × 34 in your head

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