Write a system of equations to describe the situation below, solve using substitution, and fill in the blanks. Mr. Mills and Ms. Hutchinson are teaching their classes how to write in cursive. Mr. Mills has already taught his class 4 letters. The students in Ms. Hutchinson's class, who started the unit later, currently know how to write 10 letters. Mr. Mills plans to teach his class 2 new letters per week, and Ms. Hutchinson intends to cover 1 new letter per week. Eventually, the students in both classes will know how to write the same number of letters. How many letters will the students know? How long will that take?