A bingo card contains 5 B-numbers in the range 1, 2, ..., 15;
5 I-numbers in the range 16, 17, ..., 30;
4 N-numbers in the range 31, 32, ..., 45;
5 G-numbers in the range 46, 47, ..., 60;
and 5 O-numbers in the range 61, 62, ..., 75.
The diagonal going from top-left to bottom-right contains one B-number, one I-number, no N-number because of the FREE space, one G-number, and one O-number.
There are 15 x 15 x 15 x 15 possibilities.