Question 812029
Kevin paints a house in 8 hours. Josh paints a house in 10 hours. Working together, how fast can they get the job done? 

1/8  +  1/10  =  1/x
1*(10) + 1*(8) = 18/(10*8)

80/18= 4.44444


I found 4.44 hours, but I'm not confident in my answer. 
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Why not?  If each of them took 10 hours, it'd be 5.
If each took 8 hours, it'd be 4.
So 8 and 10 --> between 4 & 5 hours.
80/18 is correct.
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A shortcut is product/sum --> 80/18