Answer
3 hours
Work Step by Step
custodian: 3 hours to completion (1/3 of job in an hour)
second custodian added, job takes 1.5 hours
1st custodian takes 3/2*1/3 = 3/6, or 1/2 of the job
$1-1/2 = 1/2$
2nd custodian takes 1.5 hours to do 1/2 of job, like the first custodian
Since the second custodian takes as much time as the first custodian to do half of the job, the second custodian would take 3 hours to do the complete job solo.
