P and Q can do a work in 9 days and 12 days respectively. P started the work and they work in alternate days. On which day the work will be completed.

P's per day work=$\frac{1}{9}$

Q's per day work=$\frac{1}{12}$

in 2 days they complete $\frac{1}{9}+\frac{1}{12}=\frac{4+3}{36}=\frac{7}{36}$

in 2$\times$5$\to$10 days they complete 5$\times \frac{7}{36}$$$

$\frac{35}{36}part of the work is completed$  in 10 days

$on 11 th day work will be completed$