PQID = DS10602.01 | OG 2020 Question No. 290
If S is a set of odd integers and 3 and –1 are in S, is –15 in S?
- 5 is in S.
- Whenever two numbers are in S, their product is in S.
| Source | OG 2020 |
| PQID | DS10602.01 |
| Type | Data Sufficiency |
| Topic | Number Properties |
| Sub-topic | Even/Odd |
| Difficulty | Medium |
Solution
Steps 1 & 2: Understand Question and Draw Inferences
In this question, we are given that:
- S is a set of odd integers
- The numbers 3 and -1 are in S
We need to determine
- Whether the number -15 is in S or not.
As we do not have any other information about the elements present in S, let us now analyse the individual statements.
Step 3: Analyse Statement 1
As per the information given in statement 1, the number 5 is in the set S.
- However, from this statement we cannot say whether the number -15 is present in S or not.
Hence, statement 1 is not sufficient to answer the question.
Step 4: Analyse Statement 2
As per the information given in statement 2, whenever two numbers are present in S, their product is also present in S.
We already know that 3 and -1 are present in S.
- Therefore, we can say that (3 × -1) or -3 is also present in S.
- However, we can’t say whether -15 is present in S or not.
- Note that even with the rule in statement 2, we need a 5 in set S to be able to create -15.
- But we have no idea about the presence of 5.
Hence, statement 2 is not sufficient to answer the question.
Step 5: Combine Both Statements Together (If Needed)
From statements 1 and 2, we can say
- 3, -1, -3 and 5 are all present in set S.
- Also, for any two elements present in S, their product is also present in S.
- Therefore, we can say (-3 × 5) or -15 is also present in S.
As we can determine that -15 is present in S by combining both statements, the correct answer is option C.
Takeaways:
- Be careful with translations. – “Set S is a set of odd integers” does NOT mean that S has ALL the odd integers in the world. The article “a” in “a set” is important!
- Be careful not to drag statement 1 when individually analyzing statement 2. This is a common mistake in DS. In this question, you would have marked B if you made that mistake!
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