Question 434762
If Peggy can clean up in 12min., then she does 1/12 a job per minute.  Likewise, Butch's time is 1/15 job per minute.  Then;
1/12+1/15=1/t, t being the time it takes them working together.
Multiply both sides by 60t:
5t+4t=60
9t=60
t=60/9 minutes or 20/3 minutes.