Each side of a rectangular field diminished by 40%. By how much per cent is the area of the field diminished?

Let the Original length of the rectangle be 20 unit and breadth be 10 unit. Then
Original Area = length *breadth = 20*10 = 200 Square unit.
40% decrease in each side, then
Length = (20 - 40% of 20) = 12 unit.
Breadth = (10 - 40% of 10) = 6 unit.
Now, Area = 12 × 6 = 72 Square unit.
Decrease in area = 200 - 72 = 128 square unit.
% Decrease in Area = $$\frac{{128}}{{200}} \times 100 = 64\% $$

Mind Calculation Method:
Let the original area be 100 square unit.
100 === 40%↓(decrease in length) ==⇒ 60 === 40%↓ === (decrease in breadth) ===> 36
Diminished in area = 100 - 36 = 64%