A tank 30 m long, 20 m wide and 12 m deep is dug in a field 500 m long and 30 m wide. By how much will the level of the field rise, if the Earth dug of the tank is evenly spread over the field?

Volume of the Earth taken out = 30 x 20 x 12
= 7200 m³
Area of the remaining portion (leaving the area of dug out portion) = 470 x 30 + 30 x 10
= 14100 + 300 = 14400 m³
Let h be height to which the field is raised by spreading the Earth dug out.
Then, 14400 x h = 7200
∴ h = 7200/14400 = 0.5 m