If you weigh 121 pounds how do you lose 10 pounds in three weeks?

The first thing that you would need to figure out is how many calories is ten pounds of fat. Then with that number you can divide by how many days (in this case 21) to figure out how many calories you need to cut a day.

3500 * 10 / 21 = 1667 calories

I then would figure out what I currently consume per day. Lets say 2200 calories per day you eat a day. I would then divide that 1667 by 2 to get 833.5. I then would cut my caloric intake by 833.5.

2200 - 833.5 = 1365.5

So I would eat 1365.5 calories and make sure that I expend 833.5 calories a day through exercise.

Note: 10 pounds in 3 weeks is a lot of weight to be losing. The recommendation is 2 pounds a week. Make sure that you consult a doctor or nutritionist to make sure that you are consuming enough calories per day.