19. The length and breadth of a rectangular piece of land are in ratio of 5:3. The owner spent Rs. 3000 for surrounding it from all the sides at Rs. 7.50 per metre. The difference between its length and breadth is:

20. A verandah 40 metres long 15 metres broad is to be paved with stones each measuring 6 dm by 5 dm. the number of stones required is:

21. Area of four walls of a room is 77 square metres. The length and breadth of the room are 7.5 metres and 3.5 metres respectively. The height of the room is: