Question 537125
Job problems are a lot like the D=RT problems.  The formula is J=RT where J is the whole job, generally 1, while R is the rate of work and T is the time.<P>
A completes the job in 7 days times 10 hours = 7 * 10 = 70 hours.  So A's rate is 1/70 of the job per hour.<P>
B completes the job in 5 days times 12 hours = 5 * 12 = 60 hours.  So B's rate is 1/60 of the job per hour.<P>
Working together they combine their rates.  1/60 + 1/70 = (60+70)/4200 = 130/4200 = 13/420.<P>
1 = 13/420*T<P>
That will give the time, in hours, it takes them to complete the job while working together.  The question seems incomplete, because it says "working together   hours a day."  What is that blank?  How many hours a day do they work together?  Whatever it is, divide the answer we find (total hours) by whatever is in that blank to find the number of days.<P>
Divide both sides by 13/420.<P>
420/13 = T = 416 and 4/13 hours.

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