This is the scene where Lady Macbeth reads the letter and determines that Macbeth needs her help becoming king. She creates the plan to kill the king, not him, which is Shakespear's way of showing she is strong and intelligent.
The number doubles every month so it'd be 32 in May, 64 in June, 128 in July, 256 in August, 512 in in September, 1024 in October, 2048 in November and 4096 in December! Adding that all together will equal 8190, I believe.
