Let's solve the given equations to find the values of x, y, and z:

Given:
x + y = 10 ---(1)
y + z = 20 ---(2)
x + z = 30 ---(3)

We can rearrange equation (1) to solve for x:
x = 10 - y ---(4)

We can rearrange equation (2) to solve for z:
z = 20 - y ---(5)

We can rearrange equation (3) to solve for x:
x = 30 - z ---(6)

Now, let's substitute the values of x and z from equations (4) and (5) into equation (6):
10 - y = 30 - (20 - y)

Simplifying further:
10 - y = 30 - 20 + y

Combining like terms:
10 - y = 10 + y

Adding y to both sides:
10 = 10 + 2y

Subtracting 10 from both sides:
0 = 2y

Dividing both sides by 2:
0 = y

Now we can substitute the value of y into equations (1), (2), and (3) to find the values of x and z:
x + 0 = 10
x = 10

0 + z = 20
z = 20

x + z = 30
10 + 20 = 30

So, we have x = 10, y = 0, and z = 20.

Now let's calculate the percentage of x + y with respect to x + y + z:
x + y = 10 + 0 = 10
x + y + z = 10 + 0 + 20 = 30

Percentage of x + y with respect to x + y + z:
(10 / 30) * 100 = 33.33%

So, the correct answer is "100/3%".