11. John worked for 18 days. If John worked during all 30 days, then he would receive $180. The condition "for each day he doesn't work, he pays back $9" means that he didn't receive $6 for that day and, plus he should pay $9 from his pocket. So, for each day he doesn't work, he reduces the $180 by $15 ($6+$9). Since he is left with nothing then he did not work $180:$15=12 days. So, he worked 18 for days.