En un curso hay 35 alumnos y alumnas, de los cuales 20 son hombres, 5 mujeres y 8 hombres tiene pelo rubio y el resto tiene pelo castaño. Se elige uno al azar y es hombre. ¿Cuál es la probabilidad de que tenga el pelo rubio?

ah, so ustedes and nosotros are my 'go-to' if i am using spanish mainly in the states? interesting, but still slightly confusing, haha."

it seems that you are a little confused about subject pronouns. yo=i, tú/usted=you, nosotros=we, ustedes=you(plural), ellos/ellas=they.

in the case of vosotros, it is an informal replacement for "ustedes" in spain. similar to using "you all" in english.

even though it is not generally used in conversation outside of spain, it is used many times in literature. i would suggest having at least some familiarity to that form, at least to recognize what it is.