Aconcession stand sells two sizes of popcorn. the small bag holds 20 ounces and costs $4, and the large bag holds 32 ounces and sells for $6. the concession stand sold 200 bags of popcorn and made $900 from the sale of popcorn.

X = large bag of popcorn
y = small bag of popcorn

x + y = 200...x = 200 - y
4x + 6y = 900

4(200 - y) + 6y = 900
800 - 4y + 6y = 900
-4y + 6y = 900 - 800
2y = 100
y = 50 <=== large bags sold

x + y = 200
x + 50 = 200
x = 200 - 50
x = 150 <== small bags