The problem wasn't misstated, when they aren't in order the solution isn't obvious unless you remember a bit about sums. Most interview problems, if restricted, can either be solved very quickly, or aren't interesting anymore. Either way, as it was stated, it's a fairly common interview question as another comment has pointed out.
You're probably right, although as of right now it reads "in some order", ergo there is no guarantee that i comes before i+1 for all i. It could have been updated though.
