The initial correction increases the average by 4, so for $n$ students, the total increase in scores is $4n$. The second correction increases the total score by $81 - 56 = 25$. Thus, the net increase in average due to both corrections is $\frac{4n + 25}{n} = 4 + \frac{25}{n}$, leading to $y - x = 4 + \frac{25}{n}$. Since $\frac{25}{n}$ rounds to approximately 1 for reasonable $n$, $y - x$ simplifies to about 3.