We let k be the proportionality constant for the relationship between number of hours, h and speed of the walker, s.
h = k/s
Substituting the known values,
12 = k/5
k = 60
For the second scenario,
h = k/s
Substituting the calculated value for k and the given value for speed,
h = (60)(3 miles/hour)
h = 20 hours
h = 20 hours
Therefore, it will take 20 hours to walk with a speed of 3 miles per hour.
