Because by making this assumption here, we're saying that inside our cells we have a very low concentration of S. And because we have a low amount of S substrate, not too much substrate will be bound to the enzyme. And so the majority of the enzyme will exist in its free state, not bound to the substrate. And that means the total enzyme is equal to approximately the free enzyme. Because this quantity here can be assumed to be very, very small when the concentration of S is low. And so this is equation four, e total is equal to the free enzyme concentration. And now we take equation three and we replace the free enzyme E with our total enzyme E. And so once we replace this with this, this is the equation that we can now ultimately use to basically study how the enzymes catalyze reactions inside our body.