Calculation of Number of Tiles:
To find the number of square tiles required, we need to divide the total area of the floor by the area of each tile.

Given:
Length of the floor = 18m
Width of the floor = 15m
Side of each tile = 30cm

Conversion:
1 meter = 100 centimeters
So, length of the floor = 18m = 18 x 100 cm = 1800 cm
Width of the floor = 15m = 15 x 100 cm = 1500 cm

Calculation:
Area of each tile = (Side of tile)^2
= (30 cm)^2
= 900 cm^2

Area of the floor = Length x Width
= 1800 cm x 1500 cm
= 2700000 cm^2

Number of tiles required = (Area of the floor) / (Area of each tile)
= 2700000 cm^2 / 900 cm^2
= 3000 tiles

Therefore, 3000 square tiles of side 30 cm will be required to cover the floor.

Calculation of Cost:
To find the cost of covering the floor with tiles, we need to multiply the number of tiles by the cost of each tile.

Given:
Cost of each tile = ₹2.50

Calculation:
Cost for covering the floor = (Number of tiles) x (Cost per tile)
= 3000 tiles x ₹2.50
= ₹7500

Therefore, the cost for covering the floor with tiles will be ₹7500.

Summary:
- To find the number of tiles required, we divided the total area of the floor by the area of each tile.
- The total area of the floor was calculated by multiplying the length and width of the floor.
- The area of each tile was found by squaring the side length.
- To calculate the cost, we multiplied the number of tiles by the cost per tile.
- The final cost for covering the floor with tiles was found to be ₹7500.