Step-by-step explanation:
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He can use it to make sure he lined his place values up properly
        
             
        
        
        
Answer:
Speed of boat in still water is 26km/hr
Step-by-step explanation:
Let the speed of boat in still water be x and speed of stream by y. 
Speed at which it went upstream would be (x-y) and the speed at which it went downstream would be (x+y). According to question x+y=24 and x-y=28. x=26 and y=-2.