The arithmetic sequence is .

The nth term of an arithmetic sequence is .

Here = First term of the sequence, d = common difference.

The sequence three arithmetic means between 55 and 115.

                  (Substitute )

               (Substitute )

                        (Subtract: )

Apply subtraction property of equality: a = b then .

        (Subtract 55 from each side)

        (Apply commutative property: )

                        (Apply additive inverse property: )

                                  (Subtract: )

Apply division property of equality: a = b then .

                             (Divide each side by 4)

                                  (Cancel common term)

d = 15                                     (Divide: )

The arithmetic means are 70, 85 and 100.