Mr. Schmidt and Ms. Huffman are teaching their classes how to write in cursive. Mr. Schmidt has already taught his class 10 letters. The students in Ms. Huffman's class, who started the unit later, currently know how to write 5 letters. Mr. Schmidt plans to teach his class 1 new letter per week, and Ms. Huffman 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?