See the following examples

```
i = (a, b); // stores b into i
i = a, b; // stores a into i. Equivalent to (i = a), b;
i = a, b, c; // stores a into i, discarding the unused b and c values
i = (a, b, c); // stores c into i, discarding the unused a and b values
return 1, 2, 3; // returns 3, not 1
return(1), 2, 3; // returns 3, not 1
```

Now your case is the first case so

```
a = 20+10-20 i.e. 10
```