If it takes 5 men 20 hours to brick a house, how long will it take 8 men to do the job if they work at the same rate ?

-- 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 .