It takes one person 5 hours to do the job, and the other person 7 hours to do two jobs. How much time will it take to do two job if they work together?

Question

anonymous · Answer

Let's say the first person is Bob, and the second person is John.
It takes Bob 5 hours to do one job, that is for sure.
John, however, takes 7 hours to do two jobs. We need to calculate how many hours it takes him to do one job. Do you know how to do this?

anonymous · Answer

Well, 2 jobs divided by 2 is one job. 
7 hours divided by 2 is 3.5 hours.

anonymous · Answer

So now we know that it takes Bob 5 hours for one job, and John 3.5 hours.

anonymous · Answer

Yeah, where do you go from there?

raden · Answer

hint : multiply both times, then divided by the sum of both times
the last result is your answer

anonymous · Answer

can you please do it for me and show steps?

john_es · Answer

I think this can be a solution, calculate the velocity: works done/time,
$$1/5+2/7=v\Rightarrow v=17/35$$
Then calculate the time needed to do 1 work,
$$t=1/v=35/17\ hours$$

anonymous · Answer

Thanks man, that might do it!