SOLUTION: A pet shop bought a litter of puppies for 120 pesos. All but four of them were sold, at a profit of 5 pesos each, for a total of 120 pesos. How many puppies were in the litter?

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Question 1138531: A pet shop bought a litter of puppies for 120 pesos. All but four of them were sold, at a profit of 5 pesos each, for a total of 120 pesos. How many puppies were in the litter?
Answer by ikleyn(52790)   (Show Source): You can put this solution on YOUR website!
.
Let "n" be the number of puppies originally.


Then the buying price of each of n puppies was  .


The selling price of each of sold (n-4) puppies was  .


The profit was 5 pesos for each sold puppies, i.e.


     -  = 5    pesos.


It is your basic equation to find "n".  To solve the equation, multiply both sides by n*(n-4).


    120n - 120*(n-4) = 5n*(n-4)

    120n - 120n + 480 = 5n^2 - 20n

    480 = 5n^2 - 20n

    5n^2 - 20n - 480 = 0

     n^2 - 4n  -  96 = 0

     (n-12)*(n+8)    = 0


The last equation has 2 roots 12 and -8, but only positive  n= 12 is meaningful solution to the given problem.


ANSWER.  Where were 12 puppies in the litter originally.


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