How to Schedule An Apex Job To Run Every 15 Minutes in Salesforce?

Sample Code:

YourScheduledApexClass obj = new YourScheduledApexClass();

String sch1 = '0 0 * * * ?';

System.schedule('Schedule Job1', sch1, obj);

String sch2 = '0 15 * * * ?';

System.schedule('Schedule Job2', sch2, obj);

String sch3 = '0 30 * * * ?';

System.schedule('Schedule Job3', sch3, obj);

String sch4 = '0 45 * * * ?';

System.schedule('Schedule Job4', sch4, obj);