A and B together can do a piece of work in 9 days. If A does thrice the work of B in a given time, the time A alone will take to finish the work is
A does thrice the work of B in a given time.
Let B's efficiency = $$x$$ units/day
=> A's efficiency = $$3x$$ units/day
Thus, (A+B)'s 1 day's work = $$x+3x=4x$$ units/day
=> Total work done by them in 9 days = $$4x \times 9=36x$$ units
$$\therefore$$ Time taken by A alone to finish the work = $$\frac{36x}{3x}=12$$ days
=> Ans - (D)
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