A can knit a pair of socks in 3 days. B can knit the same pair of socks in 9 days. If they are knitting together, then in how many days will they knit two pairs of socks ?

Number of pairs knit by A and B together in 1 day
$$\eqalign{ & = \left( {\frac{1}{3} + \frac{1}{9}} \right) \cr & = \frac{4}{9}{\text{ }} \cr} $$
∴ Required number of days,
$$\eqalign{ & = \left( {2 \div \frac{4}{9}} \right) \cr & = \left( {2 \times \frac{9}{4}} \right) \cr & = \frac{9}{2} \cr & = 4\frac{1}{2} \text{ days} \cr} $$