Problems of this type come down to finding each person's work rate, adding the rates, then determining how long to complete the entire job.
A completes the whole job in 3 hours, and so completes 1/3 of the job per hour.
B completes the whole job in 4 hours, and so completes 1/4 of the job per hour.
Working together they complete 1/3 + 1/4 = 7/12 of the job per hour.
1 (the whole job) = rate * time. The rate is 7/12 job per hour, so:
1=7/12 + t
Divide both sides by 7/12:
1*12/7 = t (in hours)
12/7 = t (in hours) = approximately 1.7 hours = 1 hour 42 minutes.
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