@Test
    public void testAdd(){
        Rational arg1=new Rational("2/3");
        Rational arg2=new Rational("3/5");
        assertEquals("19/15",Rational.add(arg1,arg2));
    }

"20/15" == "4/3"

// return gcd(|m|, |n|)
    private static int gcd(int m, int n) {
        if (m < 0) m = -m;
        if (n < 0) n = -n;
        if (0 == n) return m;
        else return gcd(n, m % n);
    }

    // return lcm(|m|, |n|)
    private static int lcm(int m, int n) {
        if (m < 0) m = -m;
        if (n < 0) n = -n;
        return m * (n / gcd(m, n));    // parentheses important to avoid overflow
    }