Assuming that this field is triangular, we can multiply the two shortest sides together, and then divide by 2, because that is the formula for the area of a right triangle.
So 15 times 20 is 300
300 divided by 2 is 150
So we can estimate the area to be 150 square meters.
(Upon solving further, we realize that this is a right triangle, so our initial estimation is the exact area)
