2/3 : 7/9 = 2/3 * 9/7 = 18/21 = 6/7
6 : 3/5 = 6 * 5/3 = 30/3 = 10/1 = 10
4 1/2 : 3 3/4 = 9/2 * 4/15 = 36/30 = 1 1/5
20/21 : 15 = 20/21 * 1/15 = 20 * 1 / 21 * 15 = 4 * 1 / 21 * 3 = 4/63
