Исправить ошибки в числительных

1) there are four hundreds people in the building.
2) they were waiting for their friends for half hour.
3) we bought three pizzas but later we ordered the fourth.
4) every four year they have elections.
5) this shirt doesn't fit him. he needs a size 12.
6) hundred of people greeted the celebrity.
7) people pay twenty per cents of income for taxes.
8) david accused a 30-years-old man of the robbery.
9) greg couldn't find the volume 6 in the library.
10) the ussr collapsed at the end of the twenty century.​