5/9 + 1/6 = 10/18 + 3/18 = 13/18
4/7 * 14/15 = (4*2)/(1*15) = 8/15
20/27 : 5/6 = 20/27 * 6/5 = (4*6)/(27*1) = 24/27 = 8/9
