Answer
It would take about 838 days to walk around the Earth.
Work Step by Step
The number of kilometers walked per day is (12 hours)(4 km/hour) which is 48 km/day.
The circumference of the Earth is $2\pi R = 2 \pi (6400)$ km
$\frac{2 \pi (6400) km}{48~km/day} = 838$ days.
It would take about 838 days to walk around the Earth.
