SOLUTION
The team consists of 15 seniors. Probability of choosing 1 senior becomes
[tex]\begin{gathered} \text{Probability = }\frac{\exp ected\text{ outcome}}{\text{total outcomeP}} \\ P(S)=\frac{1}{15} \end{gathered}[/tex]The team also consists of 7 juniors. Probability of choosing 3 juniors
[tex]P(J)=\frac{3}{7}[/tex]And the probability that one senior and 3 juniors will be chosen becomes
[tex]\begin{gathered} P(S\text{ and J) = P(S) }\times P(J) \\ =\frac{1}{15}\times\frac{3}{7} \\ =\frac{1}{5}\times\frac{1}{7} \\ =\frac{1}{35} \end{gathered}[/tex]Hence the answer is
[tex]\frac{1}{35}[/tex]