From a lot of 12 items containing 3 defectives, a sample of 5 items is drawn at random. Let the random variable X denote 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 $\frac { m } { n }$, where $\operatorname { gcd } ( m , n ) = 1$, then $n - m$ is equal to $\_\_\_\_$