Method 1: Because the shadow accounts for 1/3 of the small square area and 1/7 of the large square area, if the shadow area is x, the small square area is 3x and the large square area is 7x.

3x/7x=3/7

Method 2: (1÷1/3) ÷ (1÷1/7) = 3/7.

The area of a small square is three-sevenths of that of a big square.