
Gene G. answered 03/04/17
Tutor
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Retired Electrical Engineer Helping People Understand Math
Rather than memorizing every possible conversion factor, I like to just learn just one for each type of measurement.
The official conversion between English and metric dimensions is exactly 2.54 cm/inch. (It's the only one that's exact!)
List the conversion factors you'll need.
cm/km = 0.01/1000 = 0.00001 (1 cm is 0.01 m / 1 km is 1000 m)
cm/in = 2.54
in/ft = 12/1 = 12
ft/mi = 5280
min/hr = 60
Use the units to decide whether to multiply or divide.
Starting with km, convert to cm:
The conversion factor is cm/km. To convert km to cm we have to divide km by cm/km. So take the reciprocal of the factor as we wrote it and multiply. That makes the conversion factor 1/0.00001 km/cm.
That's (km/1) * (cm/km) = cm/1 = cm (Use the units like variable names!)
Values
25/1 * 1/0.00001 * 1/2.54 * 1/12 * 1/5280 * 1/60 =
(25*1*1*1*1*1) / (1 * 0.00001 * 2.54 * 12 * 5280 * 60) = 0.259
Units
km/hr * cm/km * in/cm * ft/in * mi/ft * hr/min =
(km*cm*in*ft*mi*hr) / (hr*km*cm*in*ft*min) = mi/min