koro7976  х:3+х:12=15:4
(3+12)х=3.75
15х=3.75
х= 4
это первый

tan4ik3107 1.
х/3 + х/12=15/4
(4х+х) /12=15/4
4х+х=15/4:1/12
5х=15/4*12
5х=15*3
х=45:5
х=9

2.
7х/9 - 3х/4=5/12
7х*4/36 - 3х*9/36=5/12
28х/36 - 27х/36=5/12
х/36=5/12
х=5/12:1/36
х=5/12*36
х=5*3
х=15

3.
1- 8х/15=4х/9
1= 4х/9+8х/15
1=4х*5/45 + 8х*3/45
20х/45 + 24х/45=1
44х/45=1
х=1: 44/45
х=1*45/44
х=45/44
х=1 1/44