How many calories?

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I want to lose about 10-15lbs. I'm currently 5'5 145. MFP tells me in order to reach this goal to eat 1200. I know you should add calories back in for working out but to me that does not make sense. If I work out and burn 500 cal and then go and eat something for an additional 500 calories what was the point of the workout? I know eating too low can put your body in starvation mode but I also do not want to work out just to eat. I have been tracking for 3 weeks now and stayed at 1200 most days with up to 1800 a few days on weekends. I have averaged between 3-5 days of working out( 3 days cardio and 2 days circuit weight training). I have actually gained 2 lbs.It is very frustrating. How much should I eat to actually lose?