Mathematics

The key insight is that you can only add two matrices if they have identical dimensions. When you delete rows from $X$ (a $6 \times 4$ matrix) and columns from $Y$ (a $4 \times 5$ matrix), you're looking for the largest matching dimensions that both resulting submatrices can achieve. The constraint is that the final row count must come from $X$'s available rows and the final column count must come from $Y$'s available columns—but both must be identical in size. You want to maximize the product of these dimensions while respecting each matrix's original shape.

The key insight is that you can only add two matrices if they have identical dimensions. When you delete rows from $X$ (a $6 \times 4$ matrix) and columns from $Y$ (a $4 \times 5$ matrix), you're looking for the largest matching dimensions that both resulting submatrices can achieve. The constraint is that the final row count must come from $X$'s available rows and the final column count must come from $Y$'s available columns—but both must be identical in size. You want to maximize the product of these dimensions while respecting each matrix's original shape.