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.

\par (A+B)'s one day's work = \frac{1}{12}

\par B's one day work = \frac{1}{28}                                                                       \lbrack (A+B)- (B) = A\rbrack

\par A's one day work = \frac{1}{12} - \frac{1}{28} = \frac{7-3}{84} = \frac{4}{84} = \frac{1}{21}

\par So, A can do it in 21 days

\par \underline{Shortcut Method}

\frac{12\times 28}{28-12} = \frac{336}{16} = 21                                                                                 t_{2} = \frac{T \times t_{1}}{t_{1}-T}