A product P forms in solution. Its concentration was measured at intervals of 50 s…

Step 1. The first interval. $\dfrac{0.140\ \text{mol/L} - 0.000\ \text{mol/L}}{50\ \text{s}} = 2.80 \times 10^{-3} = 0.00280\ \text{mol L}^{-1}\ \text{s}^{-1}$.

Step 2. The last interval. $\dfrac{0.344\ \text{mol/L} - 0.300\ \text{mol/L}}{50\ \text{s}} = 0.88 \times 10^{-3} = 0.00088\ \text{mol L}^{-1}\ \text{s}^{-1}$.

Step 3. The whole run. $\dfrac{0.344\ \text{mol/L} - 0.000\ \text{mol/L}}{200\ \text{s}} = 1.72 \times 10^{-3} = 0.00172\ \text{mol L}^{-1}\ \text{s}^{-1}$.