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If x and y are positive integers for which 3x + y + xy = 115, then what is x + y?

 May 14, 2021
 #1
avatar+22169 
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3x + y + xy  =  115     --->     y + xy  =  15 - 3x     --->     y(1 + x)  =  15 - 3x     --->     y  =  (15 - 3x) / (1 + x)

 

Graphing this, I get the point  (1, 56).

 

From this, we can get the value of  x + y.

 May 14, 2021
 #2
avatar+22169 
+1

Oops!

 

Electric Pavlov found that I switch the 115 to only 15; so the equation should be:  y  =  (115 -3x) / (1 + x).

However, the point remains the same.

 May 15, 2021

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