(1)
7
8 x=
21
40
7
8 x×
8
7 =
21
40 ×
8
7
x=
3
5
(2)X+
7
9 X=
4
3
16
9 X=
4
3
16
9 X×
9
16 =
4
3 ×
9
16
X=
3
4
(3)
3
4 x÷
1
6 =18
3
4 x÷
1
6 ×
1
6 =18×
1
6
3
4 x=3
3
4 x×
4
3 =3×
4
3
x=4
(1)
7
8 x=
21
40
7
8 x×
8
7 =
21
40 ×
8
7
x=
3
5
(2)X+
7
9 X=
4
3
16
9 X=
4
3
16
9 X×
9
16 =
4
3 ×
9
16
X=
3
4
(3)
3
4 x÷
1
6 =18
3
4 x÷
1
6 ×
1
6 =18×
1
6
3
4 x=3
3
4 x×
4
3 =3×
4
3
x=4