The least common multiple of 3 and 5 workers is 15 workers.it takes 3 workers 15 days to complete a job
Therefore it would take 15 workers (5 times as many workers) only 1/5th of 15 days or 3 days to complete a job. Therefore it would take 5 workers (only 1/3 as many workers) 3 times 3 days or 9 days to complete a job. Edwin