The work done by the rope on the box is equal to:
[tex]W=Fd \cos \theta[/tex]
where F is the force applied by the rope, d the distance covered by the box and [tex]\theta [/tex] is the angle between the direction of the force and of the displacement.
Using the data of the problem: [tex]F=255 N[/tex], [tex]d=15.0 m[/tex] and [tex]\theta = 60^{\circ}[/tex], we find the work:
[tex]W=(255 N)(15.0 m)(\cos 60^{\circ})=1912.5 J[/tex]
