lower bound = 12

C(20,7,2) = 12

Method of Construction: lex covering

Lower Bound: Schonheim

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