We let k be the proportionality constant for the relationship between number of hours, h and speed of the walker, s.Â
            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.Â