The area of sahara desert is 7 times the area of the gobi desert. if the sum of their areas is 4,000,000 square miles, find the area of each desert

X=area of Sahara.
y=area of the Gobi Desert.
We suggest this system equations:
x+y=4000000
x=7y
solve by susbstitution method.
(7y)+y=4,000,000
8y=4,000,000
y=4,000,000 / 8=500,000

x=7y
x=7(500,000)=3,500,000

The area of Sahara=3,500,00 miles².
The area of the Gobi Desert=500,000 miles²

To check:
Te sum of their areas is : 3,500,000 miles²+500,000 miles²=4,000,000 miles²
Te area of sahara (3,500,000 miles²) is 7 times the area of the Gobi Desert (7*500,000 miles²=3,500,000 miles²).

The answer of this question is 672 .