For a class photo, a group of students forms a single line from left

To get the correct approach, we assume we are facing the group of students in line. Therefore, the terms “left” and “right” are from our perspective.

Let’s first work from the left. Since Robert is the 15th student from the left and Christine is immediately to the left of Robert, there must be 13 students to the left of Christine (so that Christine is 14th and Robert is 15th in line from the left).

Now let’s work from the right. Since Christine is the 13th student from the right and Robert is immediately to the right of Christine, there must be 11 students to the right of Robert (so that Robert is the 12th and Christine is the 13th from the right). To recap, we have, from left to right:

13 students, Christine, Robert, 11 students

Therefore, there must be 13 + 1 + 1 + 11 = 26 students in the line for the class photo.

Answer: A
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counting from left 13 (upto chritine ) + Christine + Robert + 11
total 26 IMO

IMO 26

Robert = 15th Student, who is immediately to the right of Christine making her the 14th student.

Now Christine is 13th from the right making Robert 12th from the right.

This means that after Robert 11 more students are present.

Hence 15 + 11 = 26

Solution:

...........15 th...............Robert..................


............. Christine...............13 th............

15 th - Christine = 14 on the left.
13 th - Robert = 12 on the right.

It means 14 + 12 = 26

Answer A.

Robert from left: 15th position

Robert from right: 12th position (Christine is 13th from right and Robert is at the immediate right of her)

Total: 15 + 12 - 1 = 26

Answer A

Let R represent Robert
Let C represent Christine

Robert is the 15th student from the left
So there are 14 people (represented by X's) to the left of Robert to get: XXXXXXXXXXXXXXR

Robert is immediately to the right of Christine
In other words Christine is to the left of Robert.
So we get: XXXXXXXXXXXXXCR
Note: There are 13 people (represented by X's) to the left of Christine.

Christine is the 13th student from the right
This means there are 12 students to the right of Christine, and one of those students is Robert, which means there are 11 students to the right of Robert
So we get: XXXXXXXXXXXXXCRXXXXXXXXXXX

Total number of students = (13 people to the left of Christine) + Christine + Robert + (11 students to the right of Robert ) = 26

Answer: A

Bunuel / BrentGMATPrepNow - I understood the question as Robert to the right (side) of Christine, i.e. XXXXXRCXXXXX
and my answer was 28, which is one of the answer choices !? How to avoid this type of errors ?

Rumadi

I don't think you really need to worry about this one. You are correct that the question is worded ambiguously. Is Robert to Christine's right from her perspective or from the perspective of the photographer. GMAC would not word a question this way. You're all good!

I just used this method cause I panicked and for hygiene check. Did a fast scribbling so pardon the bad writing.