If you will reinstall your ubuntu system by any reason and  you want to reinstall all of currently installed packages,this tutorial may help you to do that easily.

Before you reinstall ubuntu,go to System–>Administration–>Synaptic Package Manager on your old system.

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Click File–>Save Markings,

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Type a name and select the location that you can use in your new system like a USB drive,then click Save.

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After the new system installed,mount the USB drive, go to System–>Administration–>Synaptic Package Manager again,and click File–>Read Markings then locate the file you saved, click Apply to start download and install all the packages you had installed before.