數學難題(請高人指教)

1.
12
─── = 3
3k-5
12=3(3k-5)
12=9k-15
9k=12+15
9k=27
k=27/9
k=3

2.
28
─── +4 = 6
3c-4
28
____=6-4
3c-4
28=2(3c-4)
28=6c-8
6c=28+8
6c=36
c=6

3.
x/3+x/4+x/5=47/6
60(x/3+x/4+x/5)=60(47/6)
20x+15x+12x=470
47x=470
x=10


4.
x/3-x/4+x/5=17/4
60(x/3-x/4+x/5)=60(17/4)
20x-15x+12x=15x17
17x=17x15
x=15x17/17
x=15

5.
x/3-x/9=5/18+x/7
126(x/3-x/9)=126(5/18+x/7)
42x-14x=35+18x
28x-18x=35
10x=35
x=3.5


6.
(x+4)/9=(x+2)/7
63[(x+4)/9]=63[(x+2)/7]
7(x+4)=9(x+2)
7x+28=9x+18
9x-7x=28-18
2x=10
x=5


7.
(5x-3)/2=(7x+1)/6
6[(5x-3)/2]=6[(7x+1)/6]
3(5x-3)=7x+1
15x-9=7x+1
15x-7x=1+9
8x=10
x=10/8
x=1.25



8.
x/6-x/14=(x+1)/18
126[x/6-x/14]=126[(x+1)/18]
21x-9x=7(x+1)
12x=7x+7
5x=7
x=7/5
x=1.4