Question 1198746
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First find how long it would take the two of them together to do the whole job.  Note I prefer a non-standard method for doing that....<br>
The least common multiple of the two given times is 60 minutes.  In 60 minutes Maria can do the task 60/30 = 2 times; Dana can do it 60/20 = 3 times.  So in 60 minutes the two together could do the task 5 times; and that means the two together can do the single task in 60/5 = 12 minutes.<br>
In this problem, Maria works for 10 minutes before Dana joins her.  In that time, the fraction of the job that Maria does is 10/30 = 1/3.<br>
So 2/3 of the job remains to be done when they start working together.  Since it takes them 12 minutes to do the whole job together, it takes them (2/3)*12 = 8 minutes to finish.<br>
ANSWER: 8 minutes to finish the job together<br>