def ascending_coin(coin):
    """Returns the next ascending coin in order.
    >>> ascending_coin(1)
    5
    >>> ascending_coin(5)
    10
    >>> ascending_coin(10)
    25
    >>> ascending_coin(2) # Other values return None
    """
    if coin == 1:
        return 5
    elif coin == 5:
        return 10
    elif coin == 10:
        return 25