How many of the students are there who could be the one assigned to 1921?

Ryan on December 15, 2019

Why is D correct? Why is C incorrect?



Irina on December 16, 2019


The question asks us how many of the students could be assigned to 1921?

Each of four years - 1921, 1922, 1923, 1924 will have exactly one student assigned to it out of six students - L M O R T Y.

The rules tell us that:

- If R is assigned to the project then O must be assigned to the year immediately prior to him.
We can thus immediately eliminate R.

-If T is assigned to the project then R must be assigned to the project, which means the only possible placements for T is:

O R T [ ]
1 2 3 4

Since we have OR block, and neither O nor R can be assigned to 1923 per rule 1, T must be assigned to 1923 if she is assigned to the project. We can thus eliminate T.

-Only L or T can be assigned to 1923.

We can see from the scenario above that if T is assigned to 1923, L could only be assigned to 1924, and if T is not assigned to a project, L must necessarily be assigned to 1923, otherwise, no student is assigned to 1923. Thus, the only possible placements for L are 1923 if T is out, or 1924 if T is in, which allows us to eliminate (C).





The remaining three students - O M and Y could all be assigned to 1921, hence (D) is the correct answer.

Let me know if this makes sense and if you have any other questions.