Well I guess that assumes the distribution of work across the workers is even.

Assuming that 80% of the workers take 20% of the work, and 20% do 80%

108000*0.8/(15*0.2) gives 3 workers making £ 28,800 per year
(working on approx 8 jobs a week every week),

The other 12 workers making £1800 per year (about 26 jobs per year)