Of the 200 envelopes carried by a mail track, 120 are from city K and the rest are from city J. Forty percent of the envelopes from city K are birthday cards, and thirty percent of the envelopes from city J are birthday cards. A random sample of 20 envelopes is randomly selected from the mail track. Find the variance of the number of birthday cards among a 20 randomly selected envelopes from this mail track.