Fraction of a total is calculated as the value of what fraction is being calculated over the total Thus fraction of a value = value/total, then you simplify the fraction. Before you calculate the fraction, make sure they are of the same units. If one is in a different unit, change the other so they are all in the same unit. Since the total (3 weeks) is in weeks, change the weeks to days There are 7 days in 1 week hence 3 weeks will be 3×7 days =21 days Hence the fraction for the 18 days will be 18 days / 21 days 3 goes into 18,6 times and into 21,7 times. Hence the simplification of 18∕21=6∕7