Two pipes X and Y can fill a tank in 6 hours and 9 hours respectively. If they are turned on alternately for one hour each starting with X, then what is the time taken to fill the tank?

Total capacity=LCM of $(6, 9)= 18$ units

 Pipe X's efficiency $=\frac{18}{6}=3$

Pipe Y's efficiency  $=\frac{18}{9}=2$

In 2 hours tank filled$= 3 + 2 = 5$ units 

In $3 x 2 = 6$hours tank filled $= 3 x 5 = 15$ units 

In 7th hour pipe X fill the tank $= 3$units 

$\therefore$In 7 hours tank filled $= 15 + 3 = 18$ units. 

Hence, 7 hours taken to filled the tank