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
2118=76