The distance between 16 poles is 480 meters. all poles are at equal distance. then what is the distance between the 5th and 9th

In the works, you had to divide 480 by 16 which the answer is 30 and you had to multiply by 4(the distance between the 5th to the 9th pole) which equals to the answer of 120

5                   6                    7                  8                   9

        30                  30                 30                30

30 x 4 = 120