A tank is 25 metres long, 12 metres wide and 6 metres deep. What is the cost of plastering its walls and bottom at the rate of 75 paise per square metre?

Consider a rectangular solid of length l, width w and height h. Then,
Total Surface Area
= 2lw + 2lh + 2wh
= 2(lw + lh + wh)
Volume = lwh
In this case, l = 25, w = 12 m and h = 6 m and all surfaces needs to be plastered except the top.
Hence, the total area to be plastered
total surface area - an area of the top face
= 2(lw + lh + wh) - lw
= lw + 2lh + 2hw
= (25 × 12) + 2 × (25 × 6) + 2 × (12 × 6)
= 300 + 300 + 144
= 744
Cost of plastering
= 744 × 75
= 55800 Paisa
= Tk. 558