15. Fill in the numbers $1, 2, 3, 4, 5$ in the following table (the letters and numbers are used to label rows, columns, or small cells, for example, D3 refers to the white small cell in row D and column 3), ensuring that the five numbers in each row and each column are all different. The following are some clues:
(1) The sum of the numbers in the gray cells in the 4th column is 9;
(2) The sum of the numbers in the gray cells in row C is 7;
(3) The sum of the numbers in the white cells in the 2nd column is 8;
(4) The sum of the numbers in the gray cells in row B is less than the sum of the numbers in the gray cells in row D. Therefore, the number in D4 is $\qquad$ -
Analysis: 【Logic】
(1) B4, D4 are $4$, $5$ respectively;
(2) C1, C3, C5 are $1, 2$, $4$ respectively:
(3) A2, C2, E2 can be $1, 2, 5$ or $1, 3, 4$. A2, C2, E2 are $1, 2, 5$, B2, D2 are $3, 4$ respectively.
From condition (4) $B 2+B 4=D 2+D 4$, $B 4=3, D 2=4, B 4=4, D 4=5$.