"Fill in each of the empty spaces with a non-zero digit so that the rows, columns and diagonals add up to the totals shown:
where the numbers at the bottom represent the sums of the columns in the 4x4 grid; those at the right represent the totals of the rows; the 9 at the lower right is the sum of the main diagonal and the 20 at the upper right represents the total of the diagonal going from lower left to upper right.
The x represents the smudged digit.
The puzzle Susan had in mind had only one solution. Can you find that solution?