Bunuel
If x is a positive two-digit integer, is x divisible by 17 ?
(1) x is divisible by 7.
(2) x has exactly 3 unique prime factors.
Given: x is a positive two-digit integer Target question: Is x divisible by 17 ?STRATEGY: Whenever there's a limited number of cases that satisfy a target question, it's useful to list all possible cases. For this question, there are only five positive two-digit integers that are divisible by 17. They are:
17, 34, 51, 68, 85This means we can rephrase the target question as follows...
REPHRASED target question: Does x equal 17, 34, 51, 68 or 85?Aside: the video below has tips on rephrasing the target question Statement 1: x is divisible by 7. When we test 17, 34, 51, 68 and 85, we see that NONE of them are divisible by 7.
So, the answer to the REPHRASED target question is
NO, x does not equal 17, 34, 51, 68 or 85Since we can answer the
target question with certainty, statement 1 is SUFFICIENT
Statement 2: x has exactly 3 unique prime factors.Let's check all values in the set {17, 34, 51, 68, 85}
17 is prime. So, it has exactly 1 prime factor.
34 = (2)(17). So, 34 has exactly 2 unique prime factors (2 & 17)
51 = (3)(17). So, 51 has exactly 2 unique prime factors (3 & 17)
68 = (2)(2)(17). So, 68 has exactly 2
unique prime factors (2 & 17)
85 = (5)(17). So, 85 has exactly 2 unique prime factors (5 & 17)
So, the answer to the REPHRASED target question is
NO, x does not equal 17, 34, 51, 68 or 85Since we can answer the
target question with certainty, statement 2 is SUFFICIENT
Answer: D
VIDEO ON REPHRASING THE TARGET QUESTION: