okay first you would want to draw out the pic then it asks how long it would take for the large pipe to fill the tank by itself. So all you need to know is how long each pipe took so , if the smaller pipe took 3 times longer than the big pipe and altogether there was nine hours used to fill the tank so, the long pipe must have taken 3 hrs to fill then if the smaller pipe took 3 times that it would be 3*3= 9 hr. so it would have taken the bigger pipe only 3 hrs to fill the tank. without the smaller pipe. but because the other pipe was small it gives the tank a disadvantage. P.S. sorry it took me so long.
