For the reaction R → P, the concentration of a reactant changes from 0.03 M to 0.02 M in 25 minutes.

Average rate of reaction $=-\frac{\Delta[\mathrm{R}]}{\Delta t}$

$=-\frac{[\mathrm{R}]_{2}-[\mathrm{R}]_{1}}{t_{2}-t_{1}}$

$=-\frac{0.02-0.03}{25} \mathrm{M} \mathrm{min}^{-1}$

$=-\frac{-0.01}{25} \mathrm{M} \mathrm{min}^{-1}$

$=4 \times 10^{-4} \mathrm{M} \mathrm{min}^{-1}$

$=\frac{4 \times 10^{-4}}{60} \mathrm{M} \mathrm{s}^{-1}$

$=6.67 \times 10^{-6} \mathrm{M} \mathrm{s}^{-1}$