a, 8/15 : x = 20/21
x = 8/15 : 20/21
x = 14/25
b, 12 - | 2.x - 1| = 7
| 2.x - 1| = 12 - 7
| 2.x - 1| = 5
suy ra | 2.x - 1| = +_ 5
* TH1: 2.x - 1 = 5
2.x = 5 + 1
2.x = 6
x = 6 : 2
x = 3
* TH2: 2.x - 1 = - 5
2.x = - 5 + 1
2.x = - 4
x = - 4 : 2
x = - 2
suy ra x thuộc { 3 , - 2 }