7 workers take 48 days to do one full job.
Together, they do (1/48) of the job per day.
Each worker does (1/7) of (1/(48) = 1/(48·7) of the job per day.
12 workers would do (12 / (48·7) = 1/(4·7) = 1/28 of a job per day.
So it would take them 28 days to do one whole job.
