6Q. Two pipes are used to fill the cistern completely. The second pipe opened one hour after the first pipe. Three hours after the first pipe has opened, there is still (9/20) of the cistern is to be filled. When the cistern is completely filled, it was found that each pipe has filled half of the cistern. How many hours would it take each one to completely filled the cistern individually?

a.) 10 hr & 8 hr
b.) 18 hr & 5.6 hr
c.) 15 hr & 4 hr
d.) 12 hr & 8 hr