Question about decryption
One would certainly hope not; if he can, then we would consider the encryption method "broken".

One of the things we expect from any encryption method is the resistance to a "known plaintext attack" (of which your question is an example); that is, even if he is given a number of plaintext/ciphertext pairs, he still is unable to decrypt another ciphertext that is encrypted with the same key.

Actually, we go father than that; we also ask encryption methods to be resistant to "chosen plaintext attacks"; that is, even if the attacker can ask that messages of his choosing be encrypted (and he then gets the resulting ciphertext), he still is unable to decrypt a target ciphertext.