Question 1134010
Edited to make more sense
A vessel can be filled by Pipe A in 4 hours and Pipe B in 8 hours.
 Each working independently. 
At 8 AM, Pipe A has started.
 At what time will the vessel be filled if Pipe B starts an hour later?
:
Let t = the filling time by pipe A
then
(t-1) = the filling time by pipe B
let the full vessel = 1
:
{{{t/4}}} + {{{((t-1))/8}}} = 1
multiply equation by 8, cancel denominators
2t + (t-1) = 8
2t + t = 8 + 1
3t = 9
t = 9/3
t = 3 hrs from 8:00 which is at 11:00  when the vessel will filled