Five key not working
How do I fix ... five / percent key not working
Question marked as
Most likely the keyboard is bad. Try cleaning the keyboard using the instructions in this Apple article making sure to keep the can of compressed air completely upright or you may damage the laptop with the cold wet propellant from the can.
How to clean the keyboard of your MacBook or MacBook Pro - Apple Support
Does an external keyboard have the "5" key working? If so, then the built-in keyboard is bad if cleaning did not help.