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

20 - (7 1/3 - 6 7/8) :3/4 - (5 1/4 - 4 21/40 ): 1 9/20= 18 8/9
1) 7 1/3 - 6 7/8= 22/3-55/8 = (176-165) /24 = 11/24
2) 5 1/4 - 4 21/40= (210 -181)/ 40 = 29/40
3) 11/24 : 3/4= 11*4/24*3= 11*1/6*3=11/18
4) 29/40 : 1 9/20 = 29/40 :29/20 = 29/40* 20/29= 2/4=1/2
5) 20-  11/18 = 19 7/18
6) 19 7/18 - 1/2 = 19 7/18 - 9/18=  18 16/18 = 18 8/9