Undefined reference

I don't know why it's getting "Undefined reference" in this code

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double calc_custo_de_agua(int consumo, const array<double,5> precos={2.79,3.61,3.92,6.71,11.86}){
double custo;
array<int,5> v;
v = consumo_em_faixas(consumo);
for(int i=0;i<5;i++){
    custo += precos[i]*v[i];
}
return custo;
}

cout << calc_custo_de_agua(consumo,precos);
consumo_em_faixas() doesn't appear to be defined.
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