Question

Greenland, the largest island in the world, is 21 times larger than Iceland. The combined area of Greenland and Iceland is 880,000 mi2. Write an equation to find the area of Greenland. (Use g to represent the area of Iceland.)

Answer 1

Greenland is x
Iceland is g

g + x = 880,000   
x = 21g

Solve both equations for x (Greenland)
x = 880,000 - g
x = 21g

Because of the transitive property, which states that for any real numbers a, b, and c: If a = b and b = c, then a = c, you can set the two things that equal x equal to each other.
21g = 880,000 - g

Then you simplify.
21g = 880,000 - g
21g + g = 880,000 -g +g
22g = 880,000