S&N question #3 on babies

@Reina Happy to help! *This is an explanation for the 3rd question in the Sufficient & Necessary Questions Lesson regarding full-term babies. It is not for the red/green color-blind question although it may appear on the message board connected to that question. The first sentence in the passage of #3 states "Normal full-term babies are all born with certain instinctive reflexes that disappear by the age of two months." I diagrammed this as: NFTB -> IRD2mo.The contrapositive of this is: not IRD2mo -> not NFTB. NFTB -> IRD2mo not IRD2mo -> not NFTB The second sentence states "Because this three-month-old baby exhibits these reflexes, this baby is not a normal full-term baby." Therefore, this baby = not IRD2mo, so we can conclude not NFTB. We used the Sufficient condition of the contrapositive to conclude the Necessary condition of the contrapositive. Since this is a parallel reasoning question, the correct answer choice will follow the same logic. Answer choice (D) states,"Because opossums have abdominal pouches..." This is diagrammed as: O -> AP. The contrapositive of this is: not AP-> not O. O -> AP not AP -> not O (D) goes on to state "...this animal lacks any such pouch, [so] this animal is not an opossum." Therefore, this animal = not AP, so we can conclude not O. The Sufficient condition of the contrapositive is used to conclude the Necessary condition of the contrapositive. This is the same chain of reasoning as used in the passage, so (D) is correct Does this make sense? Please let us know if you have any additional questions!

Got it! For the first part where you conclude NFTB, aren't you using the necessary to conclude the sufficient? (Which I know you can't do) Because you have the necessary (not NFTB) and you concluded not IRD2mo? Just want to clarify this.

Also, is there a possibility that a video can be posted diagramming the homework questions for S & N since they are difficult?

@Reina Where do you see the conclusion of NFTB? We are given: NFTB -> IRD2mo. In this case, NFTB is the Sufficient condition, and IRD2mo is the Necessary condition. Then, we reverse and negate in order to find the contrapositive, which is: not IRD2mo -> not NFTB. Here, not IRD2mo is the Sufficient condition and not NFTB is the Necessary condition of the contrapositive. We are then presented with a three-month-old baby who still exhibits the reflexes. Therefore, this baby = not IRD2mo. This is the Sufficient condition of our contrapositive. So, we can conclude the Necessary condition of our contrapositive, which is not NFTB. Does this help to clarify?

Yes I understand now! Thank you. Can you explain question 3 as well? I made another post but I don't understand how to diagram it so that I can compare it to the answer choices correctly.

@Reina, glad that helped. My account is showing this question as #3. Could you be talking about #2 (regarding koalas and deforestation)? If so, I provided an explanation on your other post. Let me know if you have questions!