If he saves $6 from every $15 he gets, then getting $75 means he saves $30.
Lets says he is paid 15 dollars every hour, and from those he saved 6 dollars. If he earned 72 dollars, that means he is paid 5 times (because 75/15 is 5). So if every time he is paid he saves $6, being paid 5 times means he saves $30 ($6 *5).