The amount of work done is equal to the force applied multiplied by the distance moved in the direction of the force.
Since Shannon and Chris push with identical force, the only difference between the two is the mass of the blocks.
The work done by Shannon is equal to the force she applied multiplied by the distance her block moved.
The work done by Chris is equal to the force he applied multiplied by the distance his block moved.
Since the force applied is the same, the only difference is the distance moved by each block.
Since Shannon's block is twice as massive as Chris', it will move half as far as Chris' block for the same amount of force applied.
Therefore, after pushing for 5 seconds, Chris did more work than Shannon.