Calculating the exact number of calories a person needs to lose weight can depend on various factors such as their body composition, metabolism, and activity level. However, as a general guideline, a safe rate of weight loss is about 0.5 to 1 kg per week.


To lose 2 kg in a month (which is a safe rate of weight loss), an 18-year-old woman who is 155 cm tall and weighs 42 kg, and exercises every day, should aim to create a daily calorie deficit of around 500 calories. This means that she would need to consume 500 fewer calories per day than what her body requires to maintain its current weight.


The exact number of calories she needs to consume to achieve a calorie deficit of 500 per day will depend on her activity level, but as a rough estimate, a sedentary 18-year-old woman with these measurements would require around 1400-1500 calories per day to maintain her weight. If she is exercising every day, she may require more calories to support her activity level, so her daily calorie intake would likely be higher.


To lose 2 kg in a month, she could aim to consume around 1000-1200 calories per day and continue to exercise regularly. However, it's important to note that consuming fewer than 1200 calories per day can be unsafe and may not provide enough nutrients to support a healthy body, so she should consult a healthcare professional before starting any weight loss program.