ashima has constructed a cubical water tank with lid for her house with each outer edge 300 cm long. she gets the inner four walls and the lid covered with square tiles of side 20 cm . find the amount she spent on tiles , if it is given that the cost of 4 tiles is rs. 160

Each edge of cubical water tank = 300 cm
Area of four walls of cubical tank = $4a^2 = 4\times {\left(300\right)}^2 = 4\times 90000 = 360000 {\mathrm{cm}}^2$
Each side of tile = 20 cm
So area of each tile = ${\left(20\right)}^2 = 400 {\mathrm{cm}}^2$
So number of tiles required to cover the four walls = $\frac{360000}{400} = 900$
Now cost of 4 tiles is Rs.160
so cost of 900 tiles = $\frac{160}{4}\times 900 = 36000$
Therefore total amount spent is Rs. 36000.