So the substrate concentration is somewhere around this value here at the beginning of that chemical reaction. Now let's compare the substrate concentration at the beginning to the Km value. Clearly the Km has a much higher value than the substrate concentration at the beginning. And so we're going to begin by making the following assumption. So when the time is approximately equal to zero at the beginning of that chemical reaction, the Km value is much greater than the concentration of that substrate. And so what that means is this sum, the Km value plus the substrate concentration is simply approximately equal to the Km value because this is much greater than this.