20-(7 1/3 -6 7/8)÷3/4-(5 1/4- 4 21/40)÷1 9/20

   20-(7 1/3 - 6 7/8) : 3/4 - (5 1/4 - 4 21/40) :1 9/20 == 20 - (22/3 - 55/8) * 4/3 - (21/4 - 181/40) * 20/29 == 20 - 11/24 * 4/3 - 29/40 * 20/29 = = 20 - 11/18 - 1/2 == 20 - 11/18 - 9/18 == 20 - 20/18 == 20 - 1 1/9 = 18 8/9