Philonumberist

x + y + 3 z = 27 \large x + y + 3z = 27

Let x , y x, y and z z be positive integers satisying the equation above, then find the number of ordered triplets ( x , y , z ) (x, y, z) .


The answer is 100.

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1 solution

Ashish Menon
Jul 25, 2016

x + y + 3 z = 27 x + y = 27 3 z x + y + 3z = 27\\ x + y = 27 - 3z

Since x , y x, y and z z are positive integers, the only permissible values of z = { 1 , 2 , 3 , , 8 } z = \{1,2,3,\cdots,8\}
When z = 1 z = 1 , x + y = 24 x + y = 24 and the number of ordered pairs ( x , y ) (x, y) satisfying this equation = 24 1 C 2 1 = 23 C 1 ^{24-1}C_{2-1} = ^{23}C_1 .
When z = 2 z = 2 , x + y = 21 x + y = 21 and the number of ordered pairs ( x , y ) (x, y) satisfying this equation = 21 1 C 2 1 = 20 C 1 ^{21-1}C_{2-1} = ^{20}C_1 .
When z = 3 z = 3 , x + y = 18 x + y = 18 and the number of ordered pairs ( x , y ) (x, y) satisfying this equation = 18 1 C 2 1 = 17 C 1 ^{18-1}C_{2-1} = ^{17}C_1 . \vdots
When z = 8 z = 8 , x + y = 3 x + y = 3 and the number of ordered pairs ( x , y ) (x, y) satisfying this equation = 3 1 C 2 1 = 2 C 1 ^{3-1}C_{2-1} = ^{2}C_1 .


So, the total number of waying of finding the ordered triplets ( x , y , z ) = 23 C 1 + 20 C 1 + 17 C 1 + + 2 C 1 = 23 + 20 + 17 + + 2 = 100 \begin{aligned} (x, y, z) & = ^{23}C_1 + ^{20}C_1 + ^{17}C_1 + \cdots + ^{2}C_1\\ & = 23 + 20 + 17 + \cdots + 2\\ & = \color{#3D99F6}{\boxed{100}} \end{aligned}

Very neat solution.

Pi Han Goh - 4 years, 10 months ago

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Thanks! :)

Ashish Menon - 4 years, 10 months ago

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