In an earlier draft of this problem, the sentence that the whole first chunk of the homework revolves around was about a book of cars, but I changed my mind at some point, thinking book of postcards sounded much better.

However, “part 5” still refers to cars at the end. The parenthetical part there should read “(for strings that include postcards but not both book and Greece)”. You may have been able to deduce that, but I’m here to say, yes, that is a typo.