please clarify std::ratio using

[qAlexKo]

I study std::ratio from STL
In test1: 1 inch + 1 cm == sum in mm
In test 2 I want to do a function which does the same, but using arbitrary numbers of inches and centimeters. I did it but somehow awkward -- I have not used std::ratio_add. How to do it more correctly, using std::ratio_add?

#include <iostream>
#include <ratio>

//sum inches and centimeters
void test1()
{
    std::cout << "-----test1-------";
    using inch = std::ratio<254, 10>;
    using cm = std::ratio<10, 1>;
    using sum = std::ratio_add<inch, cm>;
    std::cout << "t3: " << (double(sum::num) / sum::den) << '\n';
}

//The func must sum in millimeters X inches + Y centimeters
void test2(int how_many_inches, int how_many_centimeters)
{
    std::cout << "-----test2-------";
    using inch = std::ratio<254, 10>;
    using cm = std::ratio<10, 1>;
    using sum = std::ratio_add<inch, cm>;   //no use
    double inch_mm = (double(inch::num) / inch::den) * how_many_inches;
    double cm_mm = (double(cm::num) / cm::den) * how_many_centimeters;
    double sumXiYc = inch_mm + cm_mm;
    std::cout << "sumXiYc: " << sumXiYc << '\n';
}

//---------------------------------------
int main()
{
 test1();       //OK
 test2(10, 5);  //I try to sum 10 inches + 5 centimeters
}
//-----------------------------------------------------