Mr. Blair and Ms. Stevenson are teaching their classes how to write in cursive. Mr. Blair has already taught his class 4 letters. The students in Ms. Stevenson's class, who started the unit later, currently know how to write 2 letters. Mr. Blair plans to teach his class 1 new letter per week, and Ms. Stevenson intends to cover 2. new letters per week. Eventually, the students in both classes will know how to write the same number of letters. How long will that take?