Sorry, this is the one I looked at and could not find the file mentioned and then using Terminal what I copied in did nothing.
These exchanges have not helped me understand what I need to do to fix the problem as the sollution given seems to base the help on somebody who knows what they need to do and I can not follow the instructions.
Sounds like a corrupted plist (preferences) file.
Quit Numbers, then do the following:
The file indicated in the discussion linked in Ian's post is: com.apple.LaunchServices.plist
You'll find it at:
Macintosh HD > Users > yourusername > Library > Preferences > com.apple.LaunchServices.plist
With versions of OS X after 10.6, the User Library is hidden. To get to your Library, start in Finder.
Go to the Go menu, press and hold the option key, and select Library. Double click Preferences, then locate and select com.apple.LaunchServices.plist and drag that file to either the Desktop or the Trash.
Then try double-clickig a .numbers file.