ABC soup

Let a a , b b , c c be positive integers satisfying a b + b c = 518 ab+bc = 518 and a b a c = 360 ab-ac=360 . Find the largest possible value of the product a b c abc .


The answer is 1008.

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1 solution

We know that:
a , b , c a,b,c are positive numbers.
b ( a + c ) = 518 b(a+c)=518
a ( b c ) = 360 a(b-c)=360


Now lets factor 518 518 and 360 360 .
518 = 2 × 7 × 37 518=2×7×37
360 = 2 × 2 × 2 × 3 × 3 × 5 360=2×2×2×3×3×5

Values that can be obatined from 360 360 are 2 , 3 , 4 , 5 , 6 , 8 , 9 , 10 , 12 , 15 , 18 , 20 , 24 , 36 , 40 , 45 , 48 , 72 , 90 , . . . . . 2,3,4,5,6,8,9,10,12,15,18,20,24,36,40,45,48,72,90,.....

Now lets take a + c = 74 a+c=74 (Multiplying 37 × 2 37×2 ).
Now b b should be 7 7 .

Now, since b b is 7 7 .
a ( 7 c ) = 360 a(7-c)=360
Now putting any value obtained from 360 360 and keeping in mind that a a should be any number obtained from 360 360 .

We get c = 2 c=2 and a = 72 a=72 .

Therefore a = 72 a=72 , b = 7 b=7 and c = 2 c=2 .

Product of a b c = 72 × 7 × 2 = 1008 abc=72×7×2=\boxed{1008}

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