Ответ:
1) (7/15-1/6) + 2/5 = 14/30-5/30+12/30=21/30=7/10
2)(3/8-1/9)+25/36 = 27/72-8/72+50/72=69/72
3)(8/9-5/6)+2/3 = 48/54-45/54+36/54=39/54
4)(7/8-13/20) + 9/10 = 35/40-26/40+36/40=45/40=1. 5/40= 1. 1/8
5)(7/18-1/12)+5/6 = 14/36-3/36+30/36=41/36=1. 5/36
6)(3/4-8/15)+17/20 =45/60-32/60+51/60=64/60=1. 4/60=1. 1/15
(х+3)-17=-20
x+3-17=-20
х=-20+17-3
x=-20+14
x=-6
проверка:
(-6+3)-17=-20
-20=-20
-(10-b)+23,5=-40,4
-10+b+23,5=-40,4
b=-40,4+10-23,5
b=-53,9
-(10-(-53,9))+23,5=-40,4
-63,9+23,5=-40,4
-40,4=-40,4
(м+8/15)-2/15=0,8
м+8/15-2/15=4/5
м=4/5-8/15+2/15
м=0,4
(0,4+8/15)-2/15=0,8
0,8=0,8