Or, more thrillingly, are there contexts where numbers are written as decimals, but some decimals are illegal?
This is sometimes true for currencies: a price like £1.054 would normally be rounded to the nearest penny. But if it's a price per unit, you might need that extra digit
Percentages highlight an often-ignored point about rounding: if you've got a boundary, rounding to it can be very misleading.
For example, if something works 999 of 1000 times, it's not correct to say it's 100% effective. 99% is further from the true value, but shows the existence of the failure.
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