The area of desert a is four times the area of desert b. if the sum of their areas are 2,000,000 square miles, what is the area of each desert?

Let's start by turning what we know into equations. Desert a is four times the area of desert b. So a = 4*b. Desert a plus desert b is 2000000 square miles. So a+b=2million.
Take our second equation and substitute 4b in for a since we know a=4b.
(4b)+b=2,000,000
Once simplified, we see b=400,000 which means desert b is 400,000 square miles.
Then, we can substitute 400,000 in for b in either equation, I used the first.
a=4(400,000)
Simplified, we see a=1,600,000.
So desert a is 1,600,000 square miles.
To check our work, we can plug in both values to one equation.
a+b=2,000,000
(1,600,000)+(400,000)=2,000,000
Since 2,000,000 does equal 2,000,000, we know we got the right answer.