2021: When I first posted this challenge, we had to 'imagine' a vaccine had been found. Now, it's for real. Yeah! :-)
... A vaccine against the coronavirus has been found and governments around the globe agree on a plan to vaccinate every living human on the planet (imagine that!). To start with, capacity to vaccinate would be limited, with just 200 people being treated on the first day. Capacity then doubles every day until the world is safe.
After how many days would the world's population be completely vaccinated?
First solved in Switzerland by Vincent, aged 11