A1kg block is pushed with an external force of 20 newtons then slows to rest after moving 3m if the coefficient of friction between the block and the table is .5 what is the total work done by the block?

Work is the force done to an object to bring about a displacement of the object. 
when the block is being moved there are 2 forces acting on the block. 
the force applied acts on the block in one direction bringing about its displacement in that direction. Theres another force called frictional force which acts against the movement of the block in the opposite direction to the direction in which the block is moving.
The resultant force acting on the block is ;
F = force applied - frictional force 
frictional force - coefficient of friction *mg
mg - mass * gravitational acceleration
mg = 1 kg * 9.8 ms⁻²
      = 9.8 N
frictional force = 0.5 * 9.8 N
                        = 4.9 N
resultant force = 20 N - 4.9 N
                        = 15.1 N
work done = force * distance moved 
                 = 15.1 N * 3 m
                 = 45.3 J

Sorry my friend i don't know the answer. i am just in 6th