Good question, J.C. - and this is a common way that the GMAT likes to test rate problems. Because the question asks about the length of the letter (how many words long), it doesn't matter what rate we use for the second half.
From statement 2, we can determine that the first half of the letter will take 6 minutes to write at a rate of 40 words per minute. Mathematically, we can represent that as:
Rate = "Distance" / Time
40 words/minute = 1/2 of the letter / 6 minutes
6 * 40 words = 1/2 of the letter
240 words = 1/2 of the letter
480 words = the length of the letter
Had the question asked "how long will it take to type the letter", we'd need to know the rate for the second half, as we can't assume that the typist would stay at 40 words per minute for the entire letter (a phone call might have come through, or a typo would need to be fixed, etc.). That's why it's so important to pay particular attention to what each question asks - the change of one word or another could completely change the question (and the all-important correct answer).