This is a problem designed to teach dimensional analysis, unit conversion, and potentially significant figures.
Let us start with Earth’s circumference being 40,000 km. To answer the question we need to convert kilometers into days using appropriate conversion factors.
First we convert kilometers to hours using the conversion factor 4 km/h (walking speed) or to be more precise its inverse 1 hour / 4 km
Next we will convert hours into days using the conversion factor 1 day / 12 hours to get the answer.
40,000 km × (1 hour / 4 km) × (1 day / 12 hours) = 833 days
We could round the answer to ∼ 800 days leaving 1 significant figure (because 4 km/h has 1 significant figure), but you may want to ask your teacher whether you are expected to do it.
Edited to add: I noticed that you posted half a dozen similar problems. You should be able to solve them all using the same approach. If not, I would recommend to consider getting a tutor. Good luck!
