Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$$\frac{2}{3}\ \text{Lawns per hour}$$
We know that the overall rate is equal to the sum of the individual rates. Letting Mark's rate be $x$, we find: $$1=\frac{1}{3}+x \\ x=\frac{2}{3}\ \text{Lawns per hour}$$