Let the encryption of a message n blocks: x $=$ x$1||...||$xn from a cryptosystem

E in CBC mode and y $=$ y$1||...||$yn the corresponding ciphertext.

$1.$ Show that we can extract information in case of collision $($ie yi $=$ yj for i $=$ j$).$

$2.$ What is the collision probability for a block size of $64$ bits?

$3.$ For what value of n is the attack useful?