Fact 1: Pictures can tell a story. Fact 2: All storybooks have pictures. Fact 3: Some storybooks have words. If the first three statements are facts, which of the following statements must also be a fact? I: Pictures can tell a story better than words can. II: The stories in storybooks are very simple. III: Some storybooks have both words and pictures. (1 Mark)
I only
II only
III only
None of the statements is a known fact
ANSWER
Only statement III must be a fact, so the answer is (C) III only.
Official answer: CSource: ISRO's revised and final answer key of 16 February 2024
Let be the set of storybooks, the set of things that have pictures and the set of things that have words.
The second fact says every storybook has pictures, so .
The third fact says at least one storybook has words, so there is some with .
That storybook is in , so by the second fact it has pictures as well. It therefore has both words and pictures, so statement III must be a fact.
Statement I says pictures tell a story better than words can. The facts only say that pictures can tell a story. They never compare pictures with words, so I does not follow.
Statement II says the stories in storybooks are very simple. The facts say nothing about how simple or complex the stories are, so II does not follow.
Only statement III is forced by the facts, so the answer is option (C).
KEY IDEA
Combine an "all" statement with a "some" statement. The members picked out by "some" also fall under the "all" group, which gives a new "some" statement. A conclusion that brings in an idea the facts never mention (such as "better" or "simple") is not forced.
WATCH OUT FOR
Accepting statement I because it sounds reasonable. "Pictures can tell a story" does not mean they tell it better than words. In "must be a fact" questions, use only what the given facts say.
From the ISRO EC 2023 paper. Also Q.90 of the ISRO Scientist/Engineer Computer Science 2023 paper. Every question of this paper · The full paper (PDF, ISRO) · All 11 ISRO Reasoning questions, with notes
