The nth term of an arithmetic sequence is ,

where = First term of the sequence and d = common difference.

The sequence three arithmetic means between 2.5 and 12.5.

The sequence is .

The nth term of an arithmetic sequence is .

                   (Substitute )

               (Substitute )

                        (Subtract: )

Subtract 2.5 from each side.

                                      (Subtract: )

Divide each side by 4.

                                       (Divide: )

The arithmetic means are 5.0, 7.5 and 10.0.