During a math exam, Willy asks Ms. Matilda, the teacher, how much time is left. Ms. Matilda is known for being obtuse and answers that the amount of time left is 1/5 of the time already completed and that is also how much time is left, in a manner of speaking.
15 minutes. The total exam time is 90 minutes. If 15 minutes are left, 75 minutes have already passed, and one fifth of 75 is 15. However, if you follow Ms. Matilda’s hint and pay attention to only the numbers in 1/5, you get the answer of 15 minutes as well.