What is 12.5% of 640?

Question

12.5% looks awkward as a decimal but is a familiar fraction. Solve in your head. What is 12.5\% of 640? Aim: under 8 seconds.

Accepted Answer

Recognise that 12.5\% = 	frac{1}{8}, so just divide by 8: \frac{640}{8} = 80. So 12.5\% of 640 = \mathbf{80}. Divide by 8 in steps if you like: 640 	o 320 	o 160 	o 80 (three halvings). Memorising the "eighths" ladder pays off: 12.5\%, 25\%, 37.5\%, 50\%, 62.5\%, 75\%, 87.5\% are 	frac{1}{8} through 	frac{7}{8}. Takeaway. Convert nasty percentages to fractions: 12.5\% = 	frac18, 37.5\% = 	frac38, 6.25\% = 	frac1{16}. Dividing beats decimal multiplication.

What is 12.5% of 640?

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