Can you become extremely thin in 5 days?

Impossible I'd of thought...

Maybe doing something stupidly unhealthy like cutting out all food and going for runs, but you'd probably just collapse.

Yes it is possible. just stay on a hunger strike to become thin for at least 3 days in your house. You could drink Lemonade in middle if feeling dizzy or having a stomach ache. I'am sure you'll lose 5.5 kg in 3 days. IT IS PROVEN BY BABA RAMDEV from India.

You cannot become extremely thin in 5 days unless you are already slim at this point (at the time you ask the question). If you are significantly overweight at this point, it will take a lot, lot longer than 5 days to become extremely thin (even if you fasted).