C(15,9,2) = 4

Method of Construction: simple construction: multiple of smaller covering

 1  2  3  6  7  8 11 12 13 
 1  4  5  6  9 10 11 14 15 
 2  3  4  7  8  9 12 13 14 
 2  3  5  7  8 10 12 13 15