杨太太有一些红苹果和绿苹果。其中 60% 是红苹果，其余是绿苹果。红苹果比绿苹果多 192 个。杨太太卖掉了一些红苹果后，剩下的苹果中红苹果所占的比例变成了 25%。杨太太卖掉了多少个红苹果？ 答案: ___

1. Find the percentage difference between red and green apples initially.

Percentage of green apples initially = $100\% - 60\% = 40\%$
Difference in percentage = $60\% - 40\% = 20\%$

2. Calculate the initial number of red and green apples.

$20\%$ of total apples $\rightarrow 192$
$1\%$ of total apples $\rightarrow 192 \div 20 = 9.6$
Initial red apples = $60 \times 9.6 = 576$
Initial green apples = $40 \times 9.6 = 384$

3. Use the unchanged quantity (green apples) to find the new number of red apples.

The number of green apples remains unchanged at $384$.
New percentage of green apples = $100\% - 25\% = 75\%$
$75\%$ of new total $\rightarrow 384$
New percentage of red apples is $25\%$, which is $\frac{1}{3}$ of $75\%$.
New red apples = $384 \div 3 = 128$

4. Calculate the number of red apples sold.

Red apples sold = Initial red apples $-$ New red apples
$= 576 - 128 = 448$