If it takes 8 men 10 days to build a wall how long will it take 6 men?

8^10=6^x

80=6x

13.33=x

You are completing the same task so each side of the equation. This is what is called a balanced equation. Whatever you do to one side you have to do to the other.

It is not a direct ratio of 8 is to 10 as 6 is to x. This will give you the wrong answer. You are comparing each side of the equation to the task at hand. The given is that you know that each side of the equation equals each other because it performs the same task.