Kiriti's Numbers

Algebra Level 2

Suppose a , b a, b , and c c are positive integers such that

a b + b c = 130 , ab + bc = 130, a c + b c = 168 , ac + bc = 168, a b + a c = 228. ab + ac = 228.

Find a + b + c a+b+c .

This problem is posed by Kiriti M.


The answer is 31.

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1 solution

Ananya J
Oct 24, 2014

a b + b c = 130 E q 1 a c + b c = 168 E q 2 a b + a c = 228 E q 3 E q 3 E q 1 a c b c = 228 130 = 98 E q 4 E q 2 + E q 4 2 a c = 266 a c = 133 a b = 228 133 = 95 a = 95 b b c = 130 95 = 35 c = 35 b S u b s t i t u t i n g i n E q 2 95 × 35 b 2 + 35 = 168 95 × 35 + 35 b 2 = 168 b 2 b 2 = 35 × 95 133 b = 5 a = 95 5 = 19 , c = 35 5 = 7 a + b + c = 19 + 5 + 7 = 31 ab+bc=130-Eq\quad 1\\ ac+bc=168-Eq\quad 2\\ ab+ac=228-Eq\quad 3\\ \\ Eq\quad 3-Eq\quad 1\\ ac-bc=228-130=98-Eq\quad 4\\ \\ Eq\quad 2+Eq\quad 4\\ 2ac=266\\ ac=133\\ \Rightarrow ab=228-133=95\Rightarrow a=\frac { 95 }{ b } \\ \Rightarrow bc=130-95=35\Rightarrow c=\frac { 35 }{ b } \\ \\ Substituting\quad in\quad Eq\quad 2\\ \frac { 95\times 35 }{ { b }^{ 2 } } +35=168\\ \Rightarrow 95\times 35+35{ b }^{ 2 }=168{ b }^{ 2 }\\ \Rightarrow { b }^{ 2 }=\frac { 35\times 95 }{ 133 } \\ \Rightarrow b=5\\ \\ a=\frac { 95 }{ 5 } =19,c=\frac { 35 }{ 5 } =7\\ \Rightarrow a+b+c=19+5+7=31\\

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