19. I saw several houses. Most of them were quite unsuitable.
--> I saw several houses, most of which were quite unsuitable.
15. I know the woman whose son you bought that car from.
23. The car crashed into a queue of people. Four of them were killed
The car crashed into a queue of people, four of whom were killed
16. The roads were crowded with refugees. Many of them were wounded.
The roads were crowded with refugees, many of whom were wounded
18. In prison they fed us on dry bread. Most of its => IT ? was mouldy
=> In prison they fed us on fry bread most of which was mouldy
22 . They gave me four very bad tyres, one of which burst before I had driven four miles
21. He expected to pay me $2 for 12 eggs, four of which were broken.