Arrogantandy.

Selecting a student 25 years old or older means that he/she is either in the classe 25-34 or 35 older...Since these two events are mutually eclusive add the two probabilities to find P(25-34 or older than 35)=0.260+0.160=0.420

if A is the event to select a student between 18-24 then the complement event is not to select a student from the class 18-24 therefore

P(not to select a student 18-24)=1-0.573=0.427

Please study the algebra of events and revise the Ven diagrams that will help you in the study of probabilities.

Good Luck

MINOAS