For mass chmod'ing, you can use a command like the following one:
find public_html -type d -print0 | xargs -0 chmod 755
Please make sure to understand what it's doing instead of just copying and pasting it though.
Which permissions to choose depends on the group ownership of the directories. For a typical SuPHP setup, it's usually a good idea to grant access to public_html to only the account owner and to the web server's group (resulting in mode 750). The directories below then usually have 755 permissions, so that the web server can access the directories although the group ownership of the files and directories is typically that of the account owner.
Depending on the web application you're using, it can be somewhat tricky to choose permissions that are both secure and permissive enough so that everything works properly.
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