Multiply 486 × 5 the lazy way

Question

Multiplying by 5 head-on invites carrying errors. There is a cleaner route. Solve in your head. What is 486 	imes 5? Aim: under 5 seconds.

Accepted Answer

Since 5 = 	frac{10}{2}, multiply by 10 and halve, or halve first, then append a zero: 486 	imes 5 = \frac{486}{2} 	imes 10 = 243 	imes 10 = 2430. So the answer is \mathbf{2430}. Halving 486 is easy because it is even: half of 480 is 240, half of 6 is 3, giving 243. Tack on the zero and you're done. If the number were odd you'd get a ".5" after halving, which just means the final answer ends in 5 instead of 0. Takeaway. To multiply by 5, halve and shift a place. Halving is almost always faster and safer than a direct times-5.

Multiply 486 × 5 the lazy way

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