Let the number of acres of potatoes is x and the number of acres of corn is y.
Since there are 70 acres to plant, then

Since the cost of each potatoes acre is $20
Since the cost of each corn acre is $60
Then multiply x by 20 and y by 60 to find the cost of them
Since the farmer put aside $3000 to spend on them, then
Add the cost of both and equate the sum by 3000

Divide each term by 10 to simplify the inequality

Since the profit of each acre of potatoes is $150
Since the profit of each acre of corn is $50
Then to find the total profit, multiply x by 150 and y by 50, then sum the products up.

The variables are x and y
x represents the number of potatoes acres
y represents the number of corn acres
The objective function is P = 150x + 50y
The system of inequalities is
