physics
posted by liz .
A unit of area often used in measuring land areas is the hectare, defined as 104 m2. An openpit coal mine consumes 72 hectares of land, down to a depth of 29 m, each year. What volume of earth, in cubic kilometers, is removed in this time?

The volume of earth removed is a rectangular prism, the volume of which is the planar area multiplied by the depth.
V = 72 hectares * 29 m
= 72*104 m² * 29 m
= 72*104*29 m³
Now one cubic kilometer contains
(1000)³ m³
= 1,000,000,000 m³
All you need to do is to convert the previous answer into km³. 
.0207 km3