Multiply 48 × 15 with the "and a half" trick

Question

Fifteen breaks apart into a ten and a five, and the five is just half the ten. Solve in your head. What is 48 	imes 15? Aim: under 8 seconds.

Accepted Answer

Compute the times-10 part, then add half of it (that's the times-5 part): 48 	imes 15 = 48 	imes 10 + 	frac{1}{2}(48 	imes 10) = 480 + 240 = 720. So 48 	imes 15 = \mathbf{720}. The whole thing is "480, plus half of 480 is 240, total 720." Another view: 15 = 	frac{30}{2}, so 48 	imes 15 = 48 	imes 30 / 2 = 1440/2 = 720. Same answer. Takeaway. Times-15 is "times-10 plus half again." Any multiplier of the form 10 	imes 1.5 yields to this add-half pattern.

Multiply 48 × 15 with the "and a half" trick

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