1. data.h
    ...
    class data {
    public:
        void compute();

2. data.cpp
    ...
    void compute() {
         float index;
         index = ((a / 100) - (b + c) / 200) * (d + e);
    }

3. index.h
    ...
    class index {
    public:
        float index;
        void setIndex();

3. index.cpp
    #include data.cpp
    #include index.h
    ...
    void index::setIndex() {
         compute();
    }