Question 172783
art can do 1/30 job per minute  
:
bob.......1/40 job per minute
:
carl.......1/60 job per minute

those are all rates.....lets call time=t
:
t/30+t/40+t/60=1(job)
:
4t+3t+2t=120...muliply each term by 120
:
9t=120
:
{{{highlight(t=120/9=40/3)}}}or 13 and 1/3 minutes to do the entire job...