Nyah receives a chain letter and has to copy the letter and send it to 4 of her friends, who each have to copy the letter and send it to 4 other friends, and so on.Finding the number of letters sent on the nth mailing is an example of .

Each mailing sends four times as many letters as the previous mailing: 4, 16, 64, and so on. A pattern that multiplies each term by the same number is a geometric sequence. It asks for the number sent on a particular mailing, not the total across mailings, so this is a sequence rather than a series.

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