You can picture a math idea. Think of an equation, with a missing addend, as a machine.

A number goes into the machine. (input)
The machine performs an operation to that number.
The machine sends a new number out. (output)

                                Input Adding Machine
        Input          +                        =        Output

The operation performed by the Input Machine is +3.
 

5   + 3  = 8         4   + 3  = 7         3   + 3  = 6
          

 
Input Adding Machine
Input         +                       =       Output

The operation performed by the Input Machine is +7.
 

2   + 7  = 9        5   + 7  = 12        8   + 7  = 15
          
 

Remember:

Think of a math idea in a new way.

           Click on the input machine for practice.