Welcome to Android Central! Is the phone misbehaving? Android prefers to keep RAM relatively full with open apps, to make app opening and switching more efficient. 70-80% RAM usage isn't that unusual. Are you using some kind of "RAM cleaner" app to check the RAM usage? Because those apps usually use a big chunk of RAM themselves. On my Nexus 5 and Nexus 7, when I used to check RAM using apps like All-in-One Toolbox, it would report almost 90% usage. But when I closed out of that app, and then checked RAM usage in the Settings>Apps>Running menu, it would be more like 70%.
If you are finding problems with performance, then uninstall any apps you installed that you never use, and disable any preinstalled apps you never use. Go to the App Manager, select the app in question, and tap Disable (or Turn Off). Disabling a preinstalled app prevents it from opening in RAM.