The NW 1/4 of the SE 1/4 of Section 21 sold for $4.00 per square foot. What was the sales price?

Here again, begin with the section (640 acres) and divide by 4 (the SE 1/4 of the section).
640 acres / 4 = 160 (SE 1/4 of the section)
Next, divide 160 by 4 (the SW 1/4).
160 acres / 4 = 40 acres (NW 1/4 of SE 1/4 of a section)
Multiply 40 acres by the square footage in an acre (43,560 sq. ft.).
43,560 sq. ft. x 40 = 1,742,400 sq. ft.
Finally, now that it is known how many square feet are in this parcel of land, multiply this total by the price per square foot.
1,742,400 sq. ft. x $4 = $6,969,600