An initial concentration of x is larger than final concentration of x. The final will be less. We'll have less x and y at the end because some of these guys will convert to W and Z. So once again, the rate or average rate of x is negative change in concentration of x over time. Now, time is also multiplied by A. Now, for the case that A-B-C and D are one, we simply take away the A, because one times T is T. So the same thing can be said for w for Y.