$$ Number\ of\ juniors=9 $$

$$ Number\ of\ seniors=11 $$

As 1 senior is selected, so the number of seniors available for selection is

$$ Number\ of\ seniors=11-1=10 $$

$$ Total\ number\ of \ members \ available=10+9=19 $$

We have to find the probability of selecting a junior out of the remaining seniors and juniors:

$$ Probability\ of\ selecting\ a\ junior=\frac{Number\ of\ juniors}{Total\ number\ of \ members \ available}=\frac{9}{19} $$