--------------------------------                
|         | rate| time  |effort|                
--------------------------------                
| Mary    |     |   6.0 | 1.0  |                
--------------------------------                
| Sam     |     |   8.0 | 1.0  |                
--------------------------------                
| combined|     |       | 1.0  |                
--------------------------------                
Mary's rate = Mary's effort / Mary's time;      
Sam's rate = Sam's effort / Sam's time;         
combined rate = Mary's rate + Sam's rate;       
combined time = combined effort / combined rate;