A motor boat takes 12 hours to go downstream and it takes 24 hours to return the same distance. what is the time taken by boat in still water?

If t1 and t2 are the upstream and down stream times. Then time taken in still water is given by 

$\frac{2\times t1\times t2}{t1+t2}=\frac{2\times 12\times 24}{36}=16h$