Question 791864
 one pipe can fill a swimming pool in 15 hours, a second pipe requires 18 hours.
 If the first pipe is opened for 7 hours and then closed when the second pipe is opened, how long will it take the second to finish filling the pool?
:
Let t = time for the 2nd pipe to finish filling the pool
Let the full pool = 1
{{{7/15}}} + {{{t/18}}} = 1
multiply by the LCM of 15 and 18, that's 90, results:
90*{{{7/15}}} + 90*{{{t/18}}} = 90(1)
cancel the denominators
6(7) + 5t = 90
5t = 90 - 42
5t = 48
t = 48/5
t = 9.6 hrs