RO-TST 1987 Problem 3

Let $A$ be the set $A = \{ 1,2, \ldots, n\}$ . Determine the maximum number of elements of a subset $B\subset A$ such that for all elements $x,y$ from $B$ , $x+y$ cannot be divisible by $x-y$ .

Mircea Lascu, Dorel Mihet