1.Three hours is a long time to wait.
2.Something is wrong.
3.Someboby pays extra money. 時態有點怪,但不錯
4.Neither her sisters nor Mary is going to the school.
5.3000 Kms is far from here.
6.It was he discovered electricity. that/who不能省
7.Everybody is expected to do his duty.
1, 5的單複數應該可以,把時間,距離當整體。