SOLUTION: It takes Ralph 13 hours to paint a fence alone. Lisa can do the same job in 15 hours. If Ralph paints alone for 35 minutes before Lisa begins helping, how long must they work toget

Question 855396: It takes Ralph 13 hours to paint a fence alone. Lisa can do the same job in 15 hours. If Ralph paints alone for 35 minutes before Lisa begins helping, how long must they work together to finish painting the fence? Give your answer as a simplified fraction.
I've figured out that 1/13+1/15=28/195 But, what I am really having problems with is calculating in the 35 minutes Ralph does before Lisa starts helping.
Answer by JulietG(1812) (Show Source): You can put this solution on YOUR website!
(13*60) + (15*60) = 2x, where x is the completed job; hours multiplied into minutes.
780 + 900 minutes = 2x
1680 = 2x
840 minutes are required to finish 1 job.
.
Ralph starts 35 minutes alone.
Now the job requires 840 - 35 minutes, or 805 minutes.
If two are working, then 805/2 = 402 minutes each, after Ralph finishes his time.
That is 6 hours and 42 minutes.

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