SOLUTION: David can paint the outside of a house in 12 hr. Bill can paint the same house in 9 hr. How long will it take them working together to paint the house?

Click here to see ALL problems on Rate-of-work-word-problems

Question 521566: David can paint the outside of a house in 12 hr. Bill can paint the same house in 9 hr. How long will it take them working together to paint the house?
Found 2 solutions by stanbon, richwmiller:
Answer by stanbon(75887) (Show Source):
You can put this solution on YOUR website!
David can paint the outside of a house in 12 hr. Bill can paint the same house in 9 hr. How long will it take them working together to paint the house?
-----
David rate = 1/12 job/hr
---
Bill rate = 1/9 job/hr
----
Together rate = 1/x job/hr
-----
Equation:
rate + rate = together rate
1/12 + 1/9 = 1/x
9x + 12x = 9*12
21x = 9*12
7x = 9*4
x = 36/7 is approximately = 5 hrs 9 minutes
==============
Cheers,
Stan H.
==============

Answer by richwmiller(17219) (Show Source):
You can put this solution on YOUR website!
x/12+x/9=1
9x/108+12x/108=1
21x/108=1
21x=108
x=108/21
x=9*12/3*7=
x=3*12/7
x=36/7
x=5 1/7
check
36/7/12+36/7/9=1
3/7+4/7=1
1=1
ok