In problems of this nature, the usual assumption is that the time required is proportional to the work product and inversely proportional to the number of people doing the job. Here, there is no change in the amount of work, but the number of people is reduced by a factor of 12/18 = 2/3.
So, the amount of time required is expected to increase by a factor of 1/(2/3) = 3/2.
(6 weeks)×(3/2) = 9 weeks
It would have taken 9 weeks to sew the 540 dresses.
