def decrypt_tt2(k: int, ciphertext: list[tuple[int, int, int, int, int]]) \
        -> list[tuple[int, int, int, int, int]]:
    """Return the plaintext grade message when ciphertext is decrypted with key k.

    Preconditions:
        - k and ciphertext are valid inputs for decryption, based on the cryptosystem description

    >>> key = 20
    >>> c = [(19, 15, 84, 2, 6)]
    >>> decrypt_tt2(key, c)
    [(100, 96, 64, 83, 87)]
    """
    p = []
    for i in range(len(ciphertext)):
        incr = k ** (i + 1)
        n1, n2, n3, n4, n5 = [(n - incr) % 101 for n in ciphertext[i]]
        p.append((n1, n2, n3, n4, n5))
    return p