A teacher in a school for children from 11 to 16 years old sets a code number to unlock his classroom door. He has a method for remembering his code. He uses:

• the 2 digits of his birth month reversed (for example, February would be 02 reversed to 20);

• then the age of the children in his class at the start of the year with the digits reversed;

• and finally, the date of his birthday in the month, also reversed.

Which one of the following could not be the code to unlock his door?

To answer this question, we must note down a few concepts, as we might need to use them.

We only have 12 months in a year.

We only have a maximum number of 31 days in a month.

The children’s age must be between 11 and 16.

Let’s check all of the numbers and decide which set of 2 numbers doesn’t fit our criteria:

A. December, 15, the 5th day of the month.
B. September, 13, the 12th day of the month.
C. July, 11, 13th day of the month.
D. June, 12, 42nd day of the month? Doesn’t exist.
E. November, 15, the 19th day of the month.

We use basic spatial recognition to solve the question, while also making sure to explain why we’ve chose option D.