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It takes Paul 4 hours to do a job.
It takes Peter 2 hours to do that same job.
How many such jobs could they do together in 4 hours?
Let t = time when both of them to do one job
Let one completed job = 1
multiply by 4
t + 2t = 4
3t = 4
hr for both of them to do one job
Find how many jobs they could do in 4 hrs
4 divided by
invert the dividing fraction and multiply
= 3 jobs in 4 hrs