C(21,5,2) <=21

Method of construction: cyclic covering found by search program

 1  2  5 15 17
 2  3  6 16 18
 3  4  7 17 19
 4  5  8 18 20
 5  6  9 19 21
 1  6  7 10 20
 2  7  8 11 21
 1  3  8  9 12
 2  4  9 10 13
 3  5 10 11 14
 4  6 11 12 15
 5  7 12 13 16
 6  8 13 14 17
 7  9 14 15 18
 8 10 15 16 19
 9 11 16 17 20
10 12 17 18 21
 1 11 13 18 19
 2 12 14 19 20
 3 13 15 20 21
 1  4 14 16 21