Question 886219
One factory takes four days to produce an airplane, another takes five, and a 
third takes seven days. How long does it take the three factories combined to 
produce eighty-three airplanes? 
Let "x" be the number of days.
Rate for each factory:: 
1/4 plane per day
1/5 plane per day
1/3 plane per day
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Equation:
(1/4)x + (1/5)x + (1/3)x = 83
Multiply thru by 60 to get:
15x + 12x + 20x = 83*60
47x = 83*60
x = 105.96 days
Cheers,
Stan H.