What was the question for this implicit rationale, and corresponding answer?
Let S be the number of people on the first hundred years’ ship. The second hundred years’ ship had twice as many as the first, so it had 2S people. The third hundred years’ ship had twice as many as the second, so it had 2 * 2S = 4S people. All the ships had S + 2S + 4S = 7S = 847 people. Thus, the ship that the monster ate in the first hundred years had S = 847 / 7 = 121 people on it.
 The answer: 121
The question: A deep-sea monster rises from the waters once every hundred years to feast on a ship and sate its hunger. Over three hundred years, it has consumed 847 people. Ships have been built larger over time, so each new ship has twice as many people as the last ship. How many people were on the ship the monster ate in the first hundred years?