A and B together complete a work in 12 days. B alone can complete the work in 28 days. A alone can do the work in how many days.

(A+B)’s one day’s work = $\frac{1}{12}$ B’s one day work = $\frac{1}{28}$                                                        $\lbrack (A+B)- (B) = A\rbrack $ A’s one day work = $\frac{1}{12}$ – $\frac{1}{28}$ = $\frac{7-3}{84}$ = $\frac{4}{84}$

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