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Two seniors, Abby and Ben, and two juniors, Cathy and Dave, are to be assigned to the 3 lockers shown above according to the following rules.

• All 3 lockers are to be assigned.
• Abby and Ben cannot share a locker with each other.
• A senior cannot share a locker with a junior.

The locker assignments of all four students can be determined from the assignments of which of the following pairs?

• (A) I only
• (B) II only
• (C) III only
• (D) I and II only    CORRECT ANSWER
• (E) I, II, and III

Explanation:

Since the students are to be assigned to the lockers shown, each "assignment" is the pairing of a student with a specific locker (locker #46, #47, or #48), not the pairing of a student with another student. The conditions of the problem allow you to deduce which students will share a locker, but they are not enough to allow you to deduce the specific locker assignments. For example, knowing that Cathy and Dave will share a locker does not tell you to which locker they will be assigned.

First consider what knowing the assignments of Abby and Ben will tell you about the locker assignments of the remaining two students. Since Abby and Ben are seniors and they cannot share a locker with each other or with any juniors, you know that Cathy and Dave must share the third locker. Since you know the specific locker assignments of all four students, (I) is correct.

If you know the assignments of Ben and Cathy, you know that Abby is in the third locker and Dave must share Cathy’s locker. Therefore, (II) is correct.

If you know the assignments of Cathy and Dave (they must share the same locker), you only know to whom one of the lockers is assigned. You will not know specifically to which lockers Abby and Ben are assigned — you will only know that they do not share a locker. The correct answer is choice (D).

(from the October 16, 1999 test)