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1.) <---------------  crate ------------- >
  -10 units      +        10 units 
            = 0
Since the sum of the forces exerted by the boy and girl is 0, then there is NO net force. The object will stay at rest.
That is if, the forces are on opposite directions.

2.) If the forces are on the same directions, the this is what happens:
 ---------------- >  
 ---------------- >  
10 units + 10 units
 = 20 units
The sum of all the forces acting of the crate is 20 units, then the net force is also 20 units. The object will move from rest.