How to find index and value of an Array

Array("Tier 1", "Tier 2", "Tier 3", "Tier 4", "Tier 5", "Tier 1/2", "Tier 2/3", "Tier 3/4", "Tier 4/5")

Given an index, how do I find the value of an Array at the index?
(ex: Array(4) = ?)
And

Give a value in an Array, how do I find that value's index?
(ex: Array(?) = Tier 4)