Compute 423 × 45 in your head

Question

Solve in your head, no paper. What is 423 	imes 45? Aim: under 20 seconds.

Accepted Answer

Split at the round boundary: 423 = 400 + 23. - 400 	imes 45 = 18000, free. - 23 	imes 45: 23 	imes 40 = 920, plus 23 	imes 5 = 115, total 1035. 18000 + 1035 = 19035. A slick alternative exploits 45 = 9 	imes 5: 423 	imes 9 = 3807 (that is 4230 - 423), then 	imes 5 = 19035. Same answer, one fewer addition if you like the times-nine trick. Takeaway. Peel the round chunk (400 	imes 45) first, or factor the small number (45 = 9 	imes 5) and use 	imes 9 = 	imes 10 - 	imes 1. Keep a single running total in words: "eighteen thousand... nineteen oh thirty-five."

Compute 423 × 45 in your head

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