You can use the following apex code snippet in order to schedule your job to run every 15 minutes.
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System.schedule('Job1', '0 0 * * * ?', new scheduledMerge());
System.schedule('Job2', '0 15 * * * ?', new scheduledMerge());
System.schedule('Job3', '0 30 * * * ?', new scheduledMerge());
System.schedule('Job4', '0 45 * * * ?', new scheduledMerge());