GMAT Critical Reasoning – Common Assumption Question on Conditional Logic (Joey)

The concept of conditional statements is very important and you will come across questions using this concept often. These questions tend to be rather challenging. So let’s discuss an official GMAT question on it.

GMAT CR Conditional Statements in Assumption Questions

Question: A newborn kangaroo, or joey, is born after a short gestation period of only 39 days. At this stage, the joey’s hind limbs are not well developed, but its forelimbs are well developed, so that it can climb from the cloaca into its mother’s pouch for further development. The recent discovery that ancient marsupial lions were also born with only their forelimbs developed supports the hypothesis that newborn marsupial lions must also have needed to climb into their mothers’ pouches.

The argument in this passage relies on which of the following assumptions?

(A) All animals that are born after a short gestation period are born with some parts of their bodies underdeveloped.
(B) Well developed forelimbs would have been more advantageous to ancient marsupial lions than well developed hind limbs would have been.
(C) If the newborn marsupial lion did not climb into its mother’s pouch, then paleontologists would be able to find evidence of this fact.
(D) Newborn marsupial lions that crawled into their mothers’ pouches could not have done so had they not had only their forelimbs developed at birth.
(E) Newborn marsupial lions would not have had only their forelimbs developed if this development were of no use to the marsupial lions.

Solution: Take some time to understand the argument first.

De-Constructing the GMAT Critical Reasoning Argument

“A joey has a short gestation period but its forelimbs are well developed so that it can climb into its mother’s pouch for further development.”

The argument is telling you that the joey has a short gestation period (implying that it is not properly developed when it is born). It further states that the reason the forelimbs of a joey are well developed is that it needs to climb into its mother’s pouch. Notice the use of ‘so that’; it implies reason.

“marsupial lions were also born with only their forelimbs developed so newborn marsupial lions must also have needed to climb into their mothers’ pouches”.

It further states that marsupial lions were also born with only forelimbs developed. So they must have needed to climb into their mothers’ pouches too. The argument assumes here that forelimbs were developed for a reason (the reason is that they needed to ‘climb into their mothers’ pouches’). It assumes that if the lions did not have a need for the forelimbs to be developed, the forelimbs would not have been developed. Since the forelimbs were already developed at birth, it must have been for a reason. That is, developed forelimbs necessarily imply need to climb into mother’s pouch.

Necessary Condition Format in GMAT CR Assumptions

If you want to use the structure we learned in the last post, we can say that the assumption is similar to:

“Only if the marsupial lion needs to climb into its mother’s pouch will it have well developed forelimbs.”

A – the marsupial lion needs to climb into its mother’s pouch

B – it has well developed forelimbs

In case of ‘only if,’ ‘not A implies not B’.

‘Not A implies not B’ is ‘the marsupial lion does not need to climb into its mother’s pouch implies it doesn’t have well developed forelimbs’
This is the assumption made. Let’s see which option says the same thing.

Option (E) states that “Newborn marsupial lions would not have had only their forelimbs developed if this development were of no use to the marsupial lions.”

This is equivalent to “if this development were of no use to the marsupial lions, newborn marsupial lions would not have had their forelimbs developed.”

In case of ‘if’, A implies B which means ‘there is no use of well developed forelimbs (no need to climb into mother’s pouch)’ implies ‘the forelimbs are not well developed.’

This is the assumption we discussed above. Hence E is correct.

The only source of confusion is option (D).

(D) – Newborn marsupial lions that crawled into their mothers’ pouches could not have done so had they not had only their forelimbs developed at birth.

This is not correct. Our original argument says that developed forelimbs implies crawling into mother’s pouch. It doesn’t say that crawling into mothers’ pouches implies developed forelimbs. Notice that it has been found that these lions had developed forelimbs. The argument says that this implies that they crawled into pouches.

If we were given that it has been found that the lions crawled into their mothers’ pouches and that this implies that their forelimbs must have been developed, then we could have said that the argument is assuming what option D says. Since this is not the case, option D is not correct.

It comes down to understanding conditional statements!