1. (х+20)*45=3105
(х+20)=3105:45
(х+20)=69
х=69-20
х=49
(49+20)*45=3105
3105=3105
2. Х:34=17000+204
Х:34=17204
Х=17204*34
Х=584936
584936:34=17000+204
17204=17204
3. 57*(500-х)=11457
(500-х)=11457:57
(500-х)=201
Х=500-201
Х=299
57*(500-299)=11457
11457=11457