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We've all seen those celebrity couples whose age difference is a little creepy. But how much of a difference is okay? The popular rule of thumb says that the youngest person you should date is seven years older than half your age. For instance, when Demi Moore was 42, anyone younger than 28 would be considered creepy... which didn't stop her from marrying 25-year-old Ashton Kutcher!

In this lesson, students use linear relationships to examine the May-December romance and ask whether the Half Plus Seven rule (and its inverse, Minus Seven Times Two) is a good one. When dating, how young is too young? And how old is too old?

Students will

  • Write and manipulate linear equations and inequalities in two variables
  • Find the inverse of a linear function
  • Graph a system of linear inequalities, determine constraints with a problem's context, and interpret the results
  • Reason algebraically or numerically to determine wait-times for couples outside the bounds of Half Plus Seven

Before you begin

Students should be able to write the equation of and graph a linear inequality. This lesson can be used as a context to introduce writing a function's inverse, but will progress more quickly if students are already familiar with inverse functions.

Common Core Standards

Content Standards
Mathematical Practices


Ashton Kutcher, Demi Moore, Big Fish, Tim Burton, Ewan McGregor, Columbia Pictures, James Cleveland