In order to find the probability start the construction of the possible relay team, if the team is made by any 4 swimmers then
[tex]10\cdot9\cdot8\cdot7=5040[/tex]if the first member is a girl and the other three needs to be boys, the number of possibilities are
[tex]1\cdot5\cdot4\cdot3=60[/tex]then, divide in order to find the probability
[tex]\frac{60}{5040}=\frac{1}{84}[/tex]