For the first one, if they are in an order, and in an array, you can do a binary search, O(log2 n) time - better than linear time. Perhaps the question was misstated. Also, the friend's relationship is accidentally disclosed as a son.
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.
The question states they are in an order - ah, by that, they probably mean "not ordered".
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.
yeah i noticed the "son's interview" part in first read itself - it was amusing O(n) solution for who the friend is.