Stuck on Level 2 Question 24

If you add up all four number in each example equation, you get 10:

1+3+4+2=10
2+2+2+4=10
1+0+1+8=10
3+3+3+1=10

So for the last one it would be:

7+1+2+X=10

7+1+2 is already 10, so X is 0.