Using a rounded version of a conversion ratio allows for a quick estimate of a unit conversion using mental math or paper and pencil (no calculator). show an example of a unit conversion estimation using this strategy.

One yard is 0.9144 meters, exactly. it can be convenient to approximate this as 0.90909... meters, hence 1.1 yards per meter.

Thus 220 yards can be approximated as 200 meters, for example. Or, 1760 yards (1 mile) can be approximated as 1600 meters or 1.6 km. This same approximation translates to 1 km ≈ 5/8 mile.

Translated to speed, that means

... 50 mi/h ≈ 80 km/h

For those of us who travel by car in both the US and Canada, it is convenient to be able to make rapid conversions between kilometers and miles, or between kph and mph.

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