b,4x-10=15-x

<=>4x+x-10-15=0

<=>5x-25=0

<=>5x=25

<=>x=5

c,-12+x=5x-20

<=>-12+20+x-5x=0

<=>8-4x=0

<=>-4x=-8

<=>x=2

d,7x-4=20+3x

<=>7x-3x-4-20=0

<=>4x-24=0

<=>4x=24

<=>x=6

x+15=7-6x

<=>x+6x+15-7=0

<=>7x+8=0

<=>7x=-8

<=>x=-7/8

5x-16=40+x

5x-x  =40+16

   4x  = 56

     x =14

4x-10=15-x

 4x+x=15+10

 5x=25

 x=5
-12+x=5x-20
x-5x=-20+12
-4x=-8
x=2

7x - 4= 20 + 3x
 7x-3x= 20+4
4x = 24
x=6

 X+15=7-6x
x+15-7+6x=0
7x+8=0
7x=-8
x=-8/7