Thirteen friends and their birthdays
When Gustav and Sam were planning their shared birthday party, they noticed that the eleven friends they wanted to invite, Anna, Bill, Deb, Dory, Eliza, Gaby, Jan, John, Judy, Mary, and Otto, were all born on different months, all different from theirs, and that, just like them, they were each born on a month with at least two letters in common with their own name.
Which of the friends was born in November?
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Let's start by considering Anna and Otto because they only have two letters in their names. The only months that contain A and N or O and T are January and October respectively.
Jan is next. His month can't have A and N since Anna has her birthday there, so Jan's month must have a J. The possibilities are June and July, but July doesn't contain A or N. Therefore, Jan's birthday is in June.
Now consider John. By trying the remaining months, we see that only November can be possible for his birthday.
We're lucky and the question asks for November, so we don't have to find out about the other friends to see that John has his birthday in November.