Question 16
Put one of the four letters $A, B, C, D$ and one of the four numbers $1, 2, 3, 4$ into each cell of the $4 \times 4$ table, so that:
(a) in each row and in each column, each letter and each number appears exactly once,
(b) each combination (pair) of one letter and one number appears in exactly one cell of the table.
(a) in each row and in each column, each letter and each number appears exactly once,
(b) each combination (pair) of one letter and one number appears in exactly one cell of the table.