Does anyone know why i got #17 wrong? and what would the right answer be?

There are 12 boy and13 girls in a class. if the teacher randomly chooses a student’s name out of a hat what is the probability it will be a girl

Farnesene is a sesquiterpene. how many different isomers arising from double-bond isomerizations are possible? g

Seymour is twice as old as cassandra. if 16 is added to cassandra’s age and 16 is subtracted from seymour’s age, their ages become equal. what are their present ages? show !