The key thing here is to know the combined work formula:
$$\frac{1}{A}+\frac{1}{B}=\frac{1}{A\ and\ B}$$
where A is the amount of time it takes one worker/machine/etc. to complete a task, B is the amount of time it takes a second worker/machine/etc., and A and B is the amount of time it takes both workers/machines/etc. together.
Note: This equation works with any number of workers. So if we had three workers, our equation would be
$$\frac{1}{A}+\frac{1}{B}+\frac{1}{C}=\frac{1}{A,B,\ and\ C}$$
Here we know that that Michael and Donna painted one sidewalk in 6 hours. Plugging that into our equation gives us:
$$\frac{1}{A}+\frac{1}{B}=\frac{1}{6}$$
Statement 1
We know that 1/3 of the job took Michael and Donna 2 hours to complete together. This means that the remaining 2/3 would have taken them 4 hours to complete together. However, it took Donna 8 hours to complete on her own. Let's plug that into our equation. We'll have Donna be A and Michael be B:
$$\frac{1}{8}+\frac{1}{B}=\frac{1}{4}$$
Solving for B gives us 8. So it would also take Michael 8 hours to finish 2/3 of the job. This means that he would be able to complete the full job (one sidewalk) in 12 hours. Sufficient.
Statement 2
If donna can paint 2 sidewalks in 24 hours, she can paint 1 sidewalk in 12 hours. We already know that they can paint one sidewalk in 6 hours together, so let's plug that into our equation:
$$\frac{1}{12}+\frac{1}{B}=\frac{1}{6}$$
Solving for B gives us 12. So it would take Michael 12 hours to paint one sidewalk. Sufficient.
This combined work problem was pretty straightforward because both Donna and Michael worked at the same rate, but many are significantly more complicated. This is a formula you should have memorized for test day.
