Question 717528
Best is assume the rate of each man is additive in the group.  Each man does 1/24 the rate of the whole group.  For one man to do the work alone, he would need {{{15*24=360}}} days.


A bit more,

Since the rate of one man is 1 job in 360 days, or 1/360, if 9 men do the work, their rate is {{{9*(1/360)=1/40}}} jobs per day, or 40 days to do the job.