okay so first we need to know how much work each worker can do so we have two divide the 15 hours by 6 workers.
i decided to make to hours into half hours just to make it easier
in the end every worker can do 2 hours and 30 minutes of work
but if you add in 7 more workers it will take about 1.1 hour for each worker to complete the work thus getting it done in 7 hours