The title of this short story, "The Cold Equations," refers to the cold, factual mathematical equations that were used to calculate Marilyn's fate.
Marylin was just a young girl who boarded the ship to go to a distant planet to visit her brother whom she hasn't seen for 10 years. However, her stowing away on the cruiser creates grave dangers as the ship's computer can only calculate the amount of fuel necessary to transport the ship, the pilot, and cargo, and there is no place for Marylin. Thus, it was the computer, a mere calculation, not the human who decided her fate, and she was ejected into space to die there.