@Test
public void testNameOfTensDigit() {
assertEquals("ten", ntn.nameOfTensDigit(1));
assertEquals("twenty", ntn.nameOfTensDigit(2));
assertEquals("thirty", ntn.nameOfTensDigit(3));
assertEquals("forty", ntn.nameOfTensDigit(4));
assertEquals("fifty", ntn.nameOfTensDigit(5));
assertEquals("sixty", ntn.nameOfTensDigit(6));
assertEquals("seventy", ntn.nameOfTensDigit(7));
assertEquals("eighty", ntn.nameOfTensDigit(8));
assertEquals("ninety", ntn.nameOfTensDigit(9));
}
