Answer:
I'm going to say a arithmetic sequence
Explanation:
I say this because a arithmetic sequence of numbers such that the difference between the consecutive terms is constant. Difference here means the second minus the first. For instance, the sequence 5, 7, 9, 11, 13, 15.. . is an arithmetic progression with common difference of 2. So, when Nya sent the letter to 4 friends, her friends copied and sent it to four other friends. So, the sequence would be (if it continues) 4, 8, 12, 16, 18, etc.