4 pipes can fill a reservoir in 15, 20, 30 and 60 hrs the first was opened at 6 am and 2nd at 7 am and thirds at 8 am and 4th at 9 am when will the reservoir be full?	Select the correct option.
A) 6 hrs B) 7 hrs C) 8 hrs D) 9 hrs E) 10 hrs
Explanation:   let time be t hrs after 6 am   t/15 + (t-1)/30 + (t-2)/30 + (t-3)/60 =1   t = 7 hrs   Answer: Option 7 hrs
<result>B</result>
4 pipes can fill a reservoir in 15, 20, 30 and 60 hrs the first was opened at 6 am and 2nd at 7 am and thirds at 8 am and 4th at 9 am when will the reservoir be full?