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You will be prompted to choose an Internet connection type. You must be connected to the Internet to reinstall OS X using this method. The Recovery menu will display on-screen after your Macbook is connected to the Internet. " This will open the Disk Utility menu. For most users, the name of the default startup disk is "Macintosh HD OS X." Your Macbook Pro will now erase your startup disk. When complete, your Macbook Pro will be formatted and OS X will be installed as new.

Summary:
Power on your Macbook Pro and listen for the startup sound. Press and hold the Command + R keys on your keyboard immediately after hearing the startup sound. Release the Command + R keys when the Apple logo displays on-screen. Select the option to connect the Macbook to your Wi-Fi network, or connect the Macbook to your router using an Ethernet cable. Select "Disk Utility" from the Recovery menu, then click "Continue. Select the name of your startup disk in the left pane of Disk Utility, then click on the "Erase" tab. Select "Mac OS Extended (Journaled)" from the Format dropdown menu. Type a name for your disk, then click on the "Erase" button. Close the Disk Utility window, then click on "Reinstall OS X" from the Recovery menu. Click on "Continue," then follow the on-screen instructions to reinstall OS X.