Chemistry: The Molecular Science (5th Edition)

$$\frac{\Delta [N_2O_5]}{\Delta t} = - 2 \times \frac{\Delta [O_2]}{\Delta t}$$
$$Rate = \frac{1}{2}\frac{\Delta [N_2O_5]}{\Delta t} = -\frac{1}{1}\frac{\Delta [O_2]}{\Delta t}$$ Multiplying both sides by 2: $$\frac{\Delta [N_2O_5]}{\Delta t} = - 2 \times \frac{\Delta [O_2]}{\Delta t}$$