Processes P and R will be completed in 6 hours.
 
Then, processes Y and Q can be started simultaneously. Q will be completed at the end of 13 hours and Y at the end of 20 hours.
 
After process Q, S will get over at the end of 15 hours.
 
After process S, processes T, V and X can be started. Processes T, V and X will get over at the end of 19, 18 and 22 hours respectively.
 
Now, process U can be started immediately after 20 hours and it will get over in 10 more hours i.e., at the end of 30 hours.
 
Immediately process X gets over, i.e., after 22 hours, W can be started which will be completed at the end of 31 hours.  
 
Thus, minimum time taken to complete one unit of the product is 30 hours.