You can't lose weight in 'targeted' areas, its a case of just losing overall bodyfat levels. Like said previously, sit ups are an isolation muscle work out, promoting muscle gain. Obviously doing any type of exercise will help you burn calories and therefore help you lose weight, but there are far more efficient ways of doing so, IE Running Swimming Walking Cycling.

If you have 3 weeks to lose a certain amount of weight, get your determination out and do one of these high cardio vascular activities every other day, slowly progressing to daily if you're comfortable to do so. Eating a perfectly balanced diet of fruit, meats and carbohydrates will be the key to your loss. Although I'll drop you a hint, if you want to look thinner on a specific night, cut out carbohydrates completely from your diet 3 days before said night and your body will ditch water weight.

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<3Becky