The area of the complete estate is exactly one hundred acres. To find this answer I use the following little formula,

 __________________
 \/4ab - (a + b + c)²;
 _____________________
 4

where a, b, c represent the three square areas, in any order. 
The expression gives the area of the triangle A. 
This will be found to be 9 acres. 
It can be easily proved that A, B, C, and D are all equal in area; so the answer is 26 + 20 + 18 + 9 + 9 + 9 + 9 = 100 acres.

If every little dotted square in the diagram represents an acre, this must be a correct plan of the estate, for the squares of 5 and 1 together equal 26; the squares of 4 and 2 equal 20; and the squares of 3 and 3 added together equal 18. 
Now we see at once that the area of the triangle E is 2½, F is 4½, and G is 4. 
These added together make 11 acres, which we deduct from the area of the rectangle, 20 acres, and we find that the field A contains exactly 9 acres. 
If you want to prove that B, C, and D are equal in size to A, divide them in two by a line from the middle of the longest side to the opposite angle, and you will find that the two pieces in every case, if cut out, will exactly fit together and form A.

Or we can get our proof in a still easier way. 
The complete area of the squared diagram is 12 × 12 = 144 acres, and the portions 1, 2, 3, 4, not included in the estate, have the respective areas of 12½, 17½, 9½, and 4½. 
These added together make 44, which, deducted from 144, leaves 100 as the required area of the complete estate.