Question 906025
It takes Ralph 14 hours to paint a fence alone. Lisa can do the same job in 18 hours. If Ralph paints alone for 45 minutes before Lisa begins helping, 

 Give your answer as a simplified fraction. 
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1/14=Ralph's work rate
1/18=Lisa's work rate
1/14+1/18=18/252+14/252=32/252=8/63=work rate working together
let x=time working together to finish the job
45 min=3/4 hr
..
(3/4)(1/14)+x(8/63)=1 (100% of job)
3/56+8x/63=1
cd:56*63=3528
189+448x=3528
448x=3339
=3339/448≈7.45
how long must they work together to finish painting the fence?≈7.45 hrs