1. Many scientists are working hard to rid the world of hunger/starvation.
working hard也可以换写成struggling.
2. The number of the students in our school is reduced by twenty percent this year.
或：The number of the students in our school reduces by twenty percent this year.
3. I think that it's our responsibility/duty to support good leaders.
4. If you meet new words, you can refer to a dictionary.
meet也可以换写成come across或run across等。
5. His behaviour will possibly lead to his losing the job.
或：His behaviour will possibly lead him to lose his job.