The unknown force F must have a magnitude of 40 N.
According to Newton's first law of motion, an object will continue in its state of rest or uniform motion except it is acted upon by an unbalanced force. We are told that the object is moving at constant velocity hence it must be under the action of balanced forces.
If that is the case, the missing force F must have the magnitude of 40N in the direction shown by the arrow in the image attached to this answer.
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