Here are the rewritten sentences:

1. I can't cook as well as my mother.
> My mother cooks better than I do.

2. His motorbike can run 100 km/h while my motorbike can run only 80 km/h.
> His motorbike runs faster than mine.

3. Living in the countryside is not as convenient as living in the city.
> Living in the city is more convenient than living in the countryside.

4. I can't work as fast as my friend.
> My friend works faster than I do.

5. Hung drives more carefully than his brother.
> Hung's brother drives less carefully than Hung.