15. She started working as a secretary five years ago.

>She has worked as a secretary for five years.

16. Because of his broken leg, he didn't take part in the contest yesterday.

> Because his leg was broken, he didn't take part in the contest yesterday.

17. Despite the bad weather, we went out for a picnic

> Although the weather was bad, we went out for a picnic

18. Scientists have discovered that there is water on Mars

> It has been discovered by scientists that there is water on Mars

19. I spend three hours doing my homework every day.

> It takes me three hours to do my homework every day.