int x = 42; int s = 43; x++; x += --s; Console.WriteLine(x);

170.

int x = 42;
int s = 43;
x++;
x += --s;
Console.WriteLine(x);

s=43
x++=43;

x++=--s; equivalent to x=x+(--s) which is equivalent to
x=x+s;
x=42+43;

so x=85 (This has been tested)