The probability of a sophomore attending the prom, given that they were selected from the group of sophomores, is:
P(will attend the prom|sophomore) = (number of sophomores attending the prom) / (total number of sophomores)
From the table, we see that the number of sophomores attending the prom is 10, and the total number of sophomores is 54 (10 + 17 + 27). Therefore:
P(will attend the prom|sophomore) = 10 / 54
Simplifying the fraction, we get:
P(will attend the prom|sophomore) = 5 / 27
So the probability of a sophomore attending the prom is 5/27 (18.519%).