2/3 * 5 = 10/3 = 3 1/3
7/10 * 2 = 14/10 = 1 4/10 = 1 2/5
13/20 * 5 = 13/4 = 3 1/4
20/21 : 4 = 20/21 * 1/4 = 5/21
7/8 : 3 = 7/8 * 1/3 = 7/24
15/17 : 5 = 15/17 * 1/5 = 3/17
(7/20 + 2/5) * 3 = (7/20 + 8/20) * 3 = 15/20 * 3 = 45/20 = 9/4 = 2 1/4
(2/3 - 5/18) : 7 = (12/18 - 5/18) : 7 = 7/18 : 7 = 7/18 * 1/7 = 1/18