Question

By newton third law of motion, we know that if a rocket ship pushes down on the ground, the ground will push back up on the rocket ship with twice as much force

Answer 1

By Newton's third law of motion, we know that if a rocket ship pushes down on the ground, the ground will push back up on the rocket ship with twice as much force. ... Suppose an object has 20 N of tension force pulling it upward and 40 N of gravitational force pulling it downward.

Answer 2

We don't know that at all. The 3rd law says that the REaction is opposite and EQUAL to the action.  We don't know where that "twice as much" comes from.