CONO-SUR 1991 Problem 2

Two people, $A$ and $B$ , play the following game: $A$ start choosing a positive integrer number and then, each player in it's turn, say a number due to the following rule:

If the last number said was odd, the player add $7$ to this number; If the last number said was even, the player divide it by $2$ .

The winner is the player that repeats the first number said. Find all numbers that $A$ can choose in order to win. Justify your answer.