Elizabeth weighs 120 pounds. When she exercises she uses weights to gain muscle and she loses 600 calories per session of exercise. How much will she weigh in 16 months?
Assuming Elizabeth is not gaining or losing weight from any factors other than exercise, she would lose 600 calories per session of exercise.
If she exercises regularly, let's say 3 times per week, that would be 600 calories x 3 = 1800 calories per week.
In a month (assuming 4 weeks), she would lose 1800 calories x 4 = 7200 calories.
In 16 months, she would lose 7200 calories x 16 = 115,200 calories.
To convert calories to weight, 1 pound is approximately equal to 3500 calories.
Therefore, Elizabeth would lose 115,200 calories / 3500 = 32.914 pounds.
Starting with a weight of 120 pounds, she would weigh 120 - 32.914 = <<120-32.914=87.086>>87.086 pounds in 16 months. Answer: \boxed{87.086}.