Question 133383
solve ratio's and proportions's problems when there are 3 variables? 
If it takes 16 faucets 10 hours to fill 8 tubs, how long will it take 12 faucets to fill 9 tubes?
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# of faucets and # of tubs are directly related
# of faucets and # of hours are indirectly related.
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General Equation 
f = k*t/h where f is # of faucets, t is # of tubs, h is # of hours
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16 = k*8/10
solve for "k"
k = 20
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Equation for your problem:
f = 20*t/h
12 = 20*9/h
solve for "h"
h = (20*9)/12 = 15 hours
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It will take 15 hours to fill 9 tubs with 12 faucets.
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Cheers,
Stan H.