Answer
$P(Senior~|~National~Honor~Society)=\frac{37}{87}\approx0.4253$
Work Step by Step
$N(Senior~and~National~Honor~Society)=37$
$N(Junior~and~National~Honor~Society)=30$
$N(Sophomore~and~National~Honor~Society)=20$
$N(Freshman~and~National~Honor~Society)=0$
Total number of students in the National Honor Society: $37+30+20+0=87$
Use the Conditional Probability Rule (page 288):
$P(Senior~|~National~Honor~Society)=\frac{N(Senior~and~National~Honor~Society)}{N(National~Honor~Society)}=\frac{37}{87}\approx0.4253$
