Look for a matrix that has row sums 4 and 8 and column sums 2 and s:
Matrix = [a b]
[c d]
a+b=4
c+d=8
a+c=2
b+d=s
The four equations are solvable only if s=?
Then, find two different matrices that have the correct row and column sums.
(I was able to solve the problem up to this part to find that s=10. The two examples I calculated are a=2, b=2, c=0, and d=8 for one matrix and a=1, b=3, c=1, and d=7 for the second matrix.)
Then, write down the 4 by 4 system Ax=b with x=(a,b,c,d) and make A triangular by elimination. (This is where I'm stuck. I'm not sure how to pick the order of the rows.)