-----------------------------         
|     |time |rate| distance |         
-----------------------------         
|walk |     |  4 |          |         
-----------------------------         
|ride |     | 16 |          |         
-----------------------------         
|total| 0.5 |    |          |         
-----------------------------         
walk time = (ride rate X total time)/ 
            (walk rate + ride rate);  
walk distance = walk time X walk rate;