How to schedule apex job every 15 min?

Please use below code to schedule you job every to 15 min

System.schedule('Scheduled Job 1', '0 0 * * * ?', new ScheduledClass());
System.schedule('Scheduled Job 2', '0 15 * * * ?', new ScheduledClass());
System.schedule('Scheduled Job 3', '0 30 * * * ?', new ScheduledClass());
System.schedule('Scheduled Job 4', '0 45 * * * ?', new ScheduledClass());