问题如下：
The return distributions for the S&P 500 and Nikkei for the next year are given as:
If the actual joint distribution matrix looks as follows:
Fill in the missing cells to match the original given marginal distributions.
选项：
解释：
The sum of the rows needs to match the S&P 500 marginal distribution.
In other words, the x in this column:
needs to be such that the sum of 15 + 5 + x = 25. So, x = 5%.
Continuing yields:
The same process works going across the columns.
没看懂矩阵，能详细讲解一下吗？
