main()
{
int x=20,y=35;
x = y++ + x++;
y = ++y + ++x;
printf("%d %d\n",x,y);
} ? 

While calculating the x value, x & y values are post incremented. So, the values of x & y are added and then incremented i.e. x=56, y=36

While calculating the y value, x & y values are preincremented, so x & y values are incremented and then added i.e x=57, y=37.

{ x=y++ + x++; } equal to

x=y+x;//35+20
x++; //56
y++; //36
}

y=++y + ++x; is equal to

++y;//37
++x;//57
y=y+x;//37+57
}

Output:

x=57
 y=94