The average size of farms in the united States increased from 100 acres in 1920 to 700 acres in 1980. Let be the average size x years after 1900 in what year was the average size 400 acres

The average farm size was 400 acres in 1950.

Explanation:

Number of years = 1980 - 1920 = 60

Total growth in farm size between (1920-1980) = 700 acres

Average yearly growth in farm size (x) = (700 acres - 100 acres) / 60 = 600/60 = 10 acres

Therefore:

10 acres X 30 years = 300 acres

1920 + 30years = 1950