We want to perform some changes of units, such that the number of significant figures (the ones different than zero) does not change.
a) 75 cm to ft
We know that:
1ft = 30.48cm
1 = (1ft)/(30.48cm)
Now, remember that if we multiply a quantity by one we do not affect the quantity, so we can write:
75cm = 75cm*(1ft)/(30.48cm) = 2.5 ft
b) 0.668 kg to lb
we know that:
1kg = 2.20462 lb
then:
1 = (2.20462 lb)/(1kg)
0.668 kg = 0.668 kg*(2.20462 lb)/(1kg) = 1.47 lb
c) 0.549 uL to qt
We know that:
1 uL = 3.437*10^-9 qt
1 = (3.437*10^-9 qt)/(1 uL)
Then:
0.549 uL = 0.549 uL*(3.437*10^-9 qt)/(1 uL) = 1.91*10^-9 qt
d) 9.991 mm to km
we know that:
1mm = 1*10^-6 km
then:
1 = (1*10^-6 km)/(1mm)
So we can rewrite:
9.991 mm = 9.991 mm*(1*10^-6 km)/(1mm) = 9.991*10^-6 km
e) 313 nm to mm
We have:
1nm = 1*10^-6 mm
1 = ( 1*10^-6 mm)/(1 nm)
Then:
313 nm*( 1*10^-6 mm)/(1 nm) = 3.13*10^-4 mm
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