Question 345137
20 hens housed in 3 pens will hatch 30 eggs in 80 days.
 How long will it take 30 hens in 4 pens to hatch the same number of eggs?
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What has the number of pens got to do with it? Ignore that.
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Find the number of hen-days required to hatch 30 eggs
20*80 = 1600 hen days to hatch 30 eggs
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Let d = no. of days required by 30 hens to do the same thing
30d = 1600
d = {{{1600/30}}}
d = 53{{{1/3}}} days for 30 hens to hatch 30 eggs
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A roofer takes 12 hours to shingle a roof alone.
The roofer and his apprentice work 3 hours, then the roofer quits. 
The apprentice takes another 12 hours to finish the roof.
How long would it take the apprentice alone? 
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Let a = time required by the apprentice alone
Let the completed job = 1
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The apprentice works a total of 15 hrs, therefore the equation is:
{{{3/12}}} + {{{15/a}}} = 1
multiply eq by 12a, results:
3a + 12(15) = 12a
180 = 12a - 3a
180 = 9a
a = {{{180/3}}}
a = 20 hrs alone for the apprentice
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Check solution
{{{3/12}}} + {{{15/20}}} =
reduce fractions
{{{1/4}}} + {{{3/4}}} = 1; confirms our solution of a = 20