b) ta thấy : 

2031⋮3
18x3⋮3

ta có : 

18x5⋮3⇒(1+8+5+x)⋮3⇒14+x⋮3

ta có : 

Th1:

14+x=15

⇒x=15−14

⇒x=1

Th2

14+x=18

⇒x=18−14

⇒x=4

Th3:

14+x=21

⇒x=21−14

⇒x=7

Vậy x∈{1;4;7}

c) 60⋮x;140⋮x⇒x∈ƯC(60,140)

ta có : 

60=22.3.5

140=22.5.7

⇒ƯCLN(60,140)=22.5=20

⇒ƯC(60,140)={1;2;4;5;10;20}

⇒x∈{5;10;20}

d)x⋮12;x⋮15;x⋮18 ⇒x∈BC(12,15,18)

112=22.31

15=3.5

18=2.32

`=> BCNN(12,15,18) = 2^2 . 3^2 . 5 = 180`

⇒BC(12,15,18)={0; 180;360;540.........}

⇒x∈{0; 180;360;540;.....;720}