What does it take to burn a 0.45kg/1pound of fat in 3 weeks? According to this Havard piblication it is counting and burning calories.

These are some tips to help you reach your weight loss goal

•	It takes roughly 3,500 additional calories spent in physical activity to burn a pound of fat.
•	Walking or jogging uses up roughly 100 calories per mile. (Note: Your actual calorie expenditure depends on a number of factors, including your weight and pace.)
•	You’ll shed approximately a pound of fat for every 35 miles you walk, assuming your levels of food intake and other physical activity remain the same.
•	If you walk briskly (at a pace of 4 mph) for half an hour on five out of seven days, you’ll log 10 miles a week. At the end of three-and-a-half weeks, it’s possible to lose 0.45kg/1 pound if the number of calories you consume stays the same.
•	If you also cut back on the amount of food you eat by a few hundred calories a day, you can hasten the pace of your weight loss.

At the end of the day eating healthy and making better food choices with exercise, helps you achieve your weight loss goals.

For info : Harvard publication