Combined rate = Rate of 1 + Rate of 2
= 1/3 + 1/12 = 5/12
Therefore total time taken = 12/5 = 2, 2/5 hours
Mark does the job in 12 hours. Now he adds someone who can do the job in 12 hours, ie 4 times faster than himself. Thus, the rate of job being done should become 5 times faster than before, and time should decrease 5 times. Therefore, total time = 12/5 = 2, 2/5
San does the job in 3 hours. Now he adds someone who can do it at 1/4th the rate. Thus his rate increases from 1 to 1.25 and time should therefore decrease 1.25 times. Therefore total time = 3/1.25 = 2.4 = 2, 2/5
In one hour, Mark does 1/12th work, and Sam does 1/3rd = 5/12th work.
Therefore number of hours required = 12/5 = 2, 2/5
Hope that helps.
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