Butcher Shop Prices

At my butcher shop, you can buy 2 pounds of chicken thigh and 1 pound of chicken feet for 738 cents, and 3 pounds of chicken thigh and 1 pound of chicken heart for 852 cents.

For how many ordered sets of integers ( t , f , h ) (t, f, h ) , with each of the element between 0 and 100 inclusive, can you determine the exact total cost of t t pounds of chicken thigh, f f pounds of chicken feet, and h h pounds of chicken heart?

Assume that the cost per pound of any given meat is constant.


The answer is 884.

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2 solutions

Jack D'Aurizio
Apr 3, 2014

We just have to count the number of couples ( f , h ) [ 0 , 100 ] 2 (f,h)\in[0,100]^2 such that 3 f + 2 h [ 0 , 100 ] 3f+2h \in [0,100] , that is: S = f = 0 33 ( 100 3 f 2 + 1 ) . S=\sum_{f=0}^{33}\left(\left\lfloor\frac{100-3f}{2}\right\rfloor+1\right). By splitting the sum over even and odd values of f f , we get: S = 2 j = 0 16 ( 50 3 j ) = 2 ( 50 17 3 j = 0 16 j ) = 2 ( 50 17 3 8 17 ) = 2 17 ( 50 24 ) = 884. S = 2\cdot\sum_{j=0}^{16}(50-3j) = 2\cdot\left(50\cdot 17-3\sum_{j=0}^{16}j\right) = 2\cdot\left(50\cdot 17 - 3\cdot8\cdot 17\right) = 2\cdot 17\cdot(50-24) = 884.

Great job! I did my math wrong, though. :(

Finn Hulse - 7 years, 2 months ago

That's a lot better than using the other of the triangle and splitting the sum into three parts for the three classes m o d 3 \mod 3 . Great solution!

Carsten Meyer - 1 year, 1 month ago
K T
Mar 1, 2019

We can determine the price of any combination x ( 2 t + f ) + y ( 3 t + h ) x(2t+f)+y(3t+h) where x and y are integers.

The boundary conditions are 0 x 0\leq x , 0 y 0 \leq y and 2 x + 3 y 100 2x+3y \leq100 .

So we need to count the grid points that lie within (or on the boundary of) the triangle with vertices ( 0 , 0 ) , ( 50 , 0 ) , ( 0 , 33 1 3 ) (0,0), (50,0), (0,33\frac{1}{3}) .

Starting at the top row (y=33), subsequent rows have 1 , 3 , 4 , 6 , 7 , 9 , . . . 49 , 51 1,3,4,6,7,9,...49,51 gridpoints.

These numbers add up pairwise as 4 + 10 + 16 + . . . + 100 4+10+16+...+100 and we can take the sum of these as 104 × 17 2 = 884 \frac{104×17}{2}=\boxed{884}

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