Mahanath works 2 times as fast as Sahay. If Sahay can complete a job alone in 21 days, then in how many days can they together finish the job?
Let total work to be done = 21 units
Sahay can complete the work alone in 21 days
=> Sahay's efficiency = $$\frac{21}{21} = 1$$ unit/day
Mahanath works 2 times as fast as Sahay
=> Mahanath's efficiency = $$2 \times 1 = 2$$ units/day
Sahay and Mahanath 1 day's work = 1 + 2 = 3 units/day
$$\therefore$$ Time taken by them together to complete the work = $$\frac{21}{3} = 7$$ days
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