Caffmeister wrote:
If P is a set of integers and 3 is in P, is every positive multiple of 3 in P?
(1) For any integer in P, the sum of 3 and that integer is also in P.
(2) For any integer in P, that integer minus 3 is also in P.
I had difficulty with this question because of the wording, I wasn't sure what they were looking for exactly, and I didn't find the explanation in the book to be sufficient. If anyone can break it down into an easier explanation I'd apprecaite it.
If P is a set of integers and 3 is in P, is every positive multiple of 3 in P?Positive multiples of 3 are: 3, 6, 9, 12, 15, ... The question asks whether ALL these numbers are in the set P, taking into account that 3 is in this set.
(1) For any integer in P, the sum of 3 and that integer is also in P --> if \(x\) is in the set, so is \(x+3\) --> we know 3 is in P, hence \(3+3=6\) is also in, and as 6 is in so is \(6+3=9\), and so on. Which means that ALL positive multiples of 3 are in the set P. Sufficient.
Side note: above does not mean that only positive multiples of 3 are in P, there can be other numbers but we are only interested in them.
(2) For any integer in P, that integer minus 3 is also in P --> if \(x\) is in the set, so is \(x-3\) --> we know 3 is in P, hence \(3-3=0\) is also in and as 0 is in, so is \(0-3=-3\), and so on. So we are not sure whether all positive multiples of 3 are in P, all we know that there will be following numbers: 3, 0, -3, -6, -9, -12, ... Not sufficient.
Answer: A.
Hope it's clear.
I'm not very clear with this answer. If your statement 2 can state like that, how didn't you question statement 1 in the same way? It means that we're not sure about whether set P contains min number like 0, 3, 6. Set P can start from 500 for example. In that case not every multiple of 3 is in the set P. Insufficient