What is the value of x, when 10(x + 2) = 5(x + 8)?

Solve for x:
10 (x + 2) = 5 (x + 8)

Expand out terms of the left hand side:
10 x + 20 = 5 (x + 8)

Expand out terms of the right hand side:
10 x + 20 = 5 x + 40

Subtract 5 x from both sides:
(10 x - 5 x) + 20 = (5 x - 5 x) + 40

10 x - 5 x = 5 x:
5 x + 20 = (5 x - 5 x) + 40

5 x - 5 x = 0:
5 x + 20 = 40

Subtract 20 from both sides:
5 x + (20 - 20) = 40 - 20

20 - 20 = 0:
5 x = 40 - 20

40 - 20 = 20:
5 x = 20

Divide both sides of 5 x = 20 by 5:
(5 x)/5 = 20/5

5/5 = 1:
x = 20/5

The gcd of 20 and 5 is 5, so 20/5 = (5×4)/(5×1) = 5/5×4 = 4:

  x = 4