In general for a reaction,
$x_1 A+x_2 B \rightarrow y_1 C+y_2 D$
The rate expression may be expressed as
$-\frac{1}{x_1} \frac{d[A]}{d t}=-\frac{1}{x_2} \frac{d[B]}{d t}=\frac{1}{y_1} \frac{d[C]}{d t}=\frac{1}{y_2} \frac{d[D]}{d t} \text {. }$
For $2 \mathrm{~N}_2 \mathrm{O}_5 \rightarrow 4 \mathrm{NO}_2+\mathrm{O}_2$
$-\frac{1}{2} \frac{d\left[\mathrm{~N}_2 \mathrm{O}_5\right]}{d t}=\frac{1}{4} \frac{d\left[\mathrm{NO}_2\right]}{d t}=\frac{d\left[\mathrm{O}_2\right]}{d t}$