Answer:
a) add a dummy stroke to make the problem as an assignment problem of adding 5 strokes to 5 swimmers. see first attachment.
b) applying the Hungarian method.
  4.8    0   0.9   4.1   2.5
  10.3   0   9.1    1.6   8.7
  4.8    0   10.4   1.9   5.1
  2.8    0   3.2   2.1   4.7
   0    0    0     0    0 Â
Deduct the smallest element in each column from the other elements of the column.
  2    0   0    2.5   0
  7.5  0   8.2   0    6.2
  2    0   9.5   0.3   2.6
  0    0   2.3   0.5   2.2
  0    0   0    0    0 Â
Which implies: Â Â Â
  2       8.2   2.5   6.2
  7.5     9.5   0.3   2.6
  2       2.3   0.5   2.2
33.8 + 34.7 + 28.5 + 29.2 = 126.2
David = Back Stroke
tony = Breast Stroke
Chris = Butterfly
Carl = Free Style