Question 1148317
Joan takes 7 hours to clean the basement.
 It takes Mike 12 hours to do this job.
 How long would it take if they worked together?
\:
Let t = time required working together
let the completed job = 1
Each will do a fraction of the job, the two fractions add up to 1 (completed job)
{{{t/7}}} + {{{t/12}}} = 1
Get rid of the fractions, multiply by 84
84*{{{t/7}}} + 84*{{{t/12}}} = 84(1)
Cancel the denominators
12t + 7t = 84
19t = 84
t = 84/19
t = 4.421 hrs, or 4 + .421(60) ~ 4 hrs 25 min