Unable to resolve target 'android-x' - Android SDK Error

If you get one of these errors:
Unable to resolve target 'android-1'
Unable to resolve target 'android-2'
Unable to resolve target 'android-3'
Unable to resolve target 'android-4'
Unable to resolve target 'android-5'
Unable to resolve target 'android-6'
Unable to resolve target 'android-7'
Unable to resolve target 'android-8'
Unable to resolve target 'android-9'
Unable to resolve target 'android-10'
Unable to resolve target 'android-11'
Unable to resolve target 'android-12'
Unable to resolve target 'android-13'
Unable to resolve target 'android-14'
Unable to resolve target 'android-15'

Reason: 
Could not find the proper Android SDK version. If the android SDK is installed correctly the problem is that the platform SDK requested by the "default.properties" is not installed. For example:

Unable to resolve target 'android-1' - (Android 1.0) change the "default.properties"
Unable to resolve target 'android-2' - (Android 1.1) change the "default.properties"
Unable to resolve target 'android-3' - install SDK Platform Android 1.5
Unable to resolve target 'android-4' - install SDK Platform Android 1.6
Unable to resolve target 'android-5' - install SDK Platform Android 2.0
Unable to resolve target 'android-6' - install SDK Platform Android 2.0.1
Unable to resolve target 'android-7' - install SDK Platform Android 2.1
Unable to resolve target 'android-8' - install SDK Platform Android 2.2
Unable to resolve target 'android-9' - install SDK Platform Android 2.3
Unable to resolve target 'android-10' - install SDK Platform Android 2.3.3
Unable to resolve target 'android-11' - install SDK Platform Android 3.0
Unable to resolve target 'android-12' - install SDK Platform Android 3.1
Unable to resolve target 'android-13' - install SDK Platform Android 3.2
Unable to resolve target 'android-14' - install SDK Platform Android 4.0
Unable to resolve target 'android-15' - install SDK Platform Android 4.0.3

Solution:
To install the missing SDK, using Eclipse:

- Click on "Windows" > "Android SDK Manager"
- Choose the missing packages and install them.

Source: Sagi's Tech