Error "Cannot Resolve symbol R"

Question

This problem has been addressed hundreds of times here but I haven't still solved it.

I have tried everything available on Stack Overflow and everywhere else:

• Build -> Clean Project; Build -> Rebuild Project
• Tools -> Android -> Sync Project with Gradle files
• Update SDK Manager (it wasn't updated)
• Add import android.R and import android.R.*

But now I don't know what to do.

Answer 1

(Posted on behalf of the OP).

I had to update to buildToolsVersion "21.1.2".