Probability Part 2

From a lot of 12 items containing 3 defectives, a sample of 5 items is drawn at random. Let the random variable X denlote the number of defective items in the sample. Let items in the sample be drawn one by one without replacement. If variance of X is ‌

m

n

, where gcd(m,n)=1, then n−m is equal to ______.