-- 5 men take 20 hours to do 1 job .
-- 5 men do 1/20 of a job per hour .
-- 1 man does 1/(5 x 20) = 1/100 = 1% of a job per hour.
-- 8 men do 8/100 of a job per hour .
-- 8 men work at the unit rate of 8/100 job/hr, or 100/8 hr/job.
-- 100/8 hr/job = 12.5 hour per job.
8 men take 12.5 hours .
