If you mix equal parts of 30 volume developer and 10 volume wall that equal 20?

When you mix equal parts of 30 volume developer and 10 volume developer, the resulting mixture averages the volume levels. Therefore, combining these two would yield a 20 volume developer, as it effectively dilutes the stronger developer with the weaker one. This is because the formula for mixing volumes is to take the average of the two, leading to a balanced 20 volume result.