Challenge

Hi,

I have the following equation:

x1 + x2 + x3 - 2(x4 + x5 + x6) + 3(x7) = N

each x_i can take any value: 1, 2, 3, 5, 6, 10, 15 or 30 and
each one is different from each other.

Which combination of values ??in the formula which leads to 
the smallest value of N?

How can I program this situation?

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Silvano Cesar da Costa
Departamento de Estat?stica
Universidade Estadual de Londrina
Fone: 3371-4346