Example sentences for Lesson 3 Part 3

Dear native English speakers,
Would you check my sentences?

1. This group has been helping the elderly and the disabled in this area(/neighborhood /region) for many years.

2. The scientists have been working on this problem for ten years.

3. This is a typical Japanese breakfast.

4a. They no longer have to work for a long time.
4b. They don't have to work for a long time any longer. [Is this awkward?]

5a. The city provides us with safe and clean water.
5b. The city supplies us with safe and clean water.
[Do 5a and 5b have the same meaning?]

6a. Mike has been working on the difficult math problem for a week.
6b. Mike has worked on the difficult math problem for a week.
[Can 6a and 6b have the same meaning?]

7a. I'm not satisfied with his answer to my question.
7b. I don't feel satisfied with his answer to my question.

Thanks in advance.

Hirashin

Generally ok I think.

hirashin said:

4a. They no longer have to work for a long time.
4b. They don't have to work for a long time any longer. [Is this awkward?]

Click to expand...

No it's not awkward.

hirashin said:

5a. The city provides us with safe and clean water.
5b. The city supplies us with safe and clean water.
[Do 5a and 5b have the same meaning?]

Click to expand...

Yes, same.

hirashin said:

6a. Mike has been working on the difficult math problem for a week.
6b. Mike has worked on the difficult math problem for a week.
[Can 6a and 6b have the same meaning?]

Click to expand...

Basically same meaning imo. 6b implies that he may be no longer working on it.
I think it would be more natural to say "a difficult math problem" than "the difficult math problem." The latter means a specific problem. If it was such a specific problem, probably it would have a name like "Mike has been working the Pythagorean Potato Hypothesis for a week."

Thanks for the help, mdchachi.