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.
