Answer: 480 acres
Step-by-step explanation:
Let's say y is the amount of acres total.
x is the amount of days taken to plow.
Meaning, that 80x = y.
But y also equals 90(x-1)+30. They plowed 90 acres a day for one less day, and then on the last day, they only had to plow 30 acres.
Substitute out the y. 80x = 90x - 90 + 30
Simplify and solve: 80x=90x-60, 10x=60, x=10
Don't forget to multiply x by 80 for the total number of acres plowed.