P and Q can do a work in 4 hours and 12 hours respectively. P starts the work at 9am and they work alternately for one hour each. When will the work be completed ?

Work done by P and Q in the first two hours, working alternately

= First hour P + Second hour Q

$\Rightarrow \frac{1}{4}+\frac{1}{12}=\frac{1}{3}$

work is completed in 2 hours

Then, the total time required to complete the work by P and Q working alternately=2 x 3= 6hours

Thus, work will be completed at 3pm.