public void Div_Valid_ReturnFormula()
        {
            int first  = 10;
            int second = 5;

            string formula = StringFormula.Div(first, second);

            Assert.That(formula, Is.EqualTo("10 / 5 = 2"));
        }
        public void Sub_Valid_ReturnFormula()
        {
            int first  = 10;
            int second = 5;

            string formula = StringFormula.Sub(first, second);

            Assert.That(formula, Is.EqualTo("10 - 5 = 5"));
        }
        public void Mul_Valid_ReturnFormula()
        {
            int first  = 10;
            int second = 5;

            string formula = StringFormula.Mul(first, second);

            Assert.That(formula, Is.EqualTo("10 * 5 = 50"));
        }
        public void Add_Valid_ReturnFormula()
        {
            int first  = 10;
            int second = 5;

            string formula = StringFormula.Add(first, second);

            Assert.That(formula, Is.EqualTo("10 + 5 = 15"));
        }