x^2+(x/2)^2=r^2=500=>； x=20,x^2=400

That is, the square covers an area of 400 square meters, and * * * can raise 1200 crab seedlings.

Take a few crabs at random, calculate the average weight, and then estimate the total weight. Of course, crabs that may die should also be considered. This depends on the actual situation (I don't know the mortality rate of crab seedlings).

There is no picture of Monopterus albus, so let's assume it is like this. Let the side length of a small square be y, and then from the right triangle EFO, we can get:

y^2+(y+x/2)^2=r^2=500=>； y^2+(y+ 10)^2=500

Namely: (y+20) (y-10) = 0 = > y =10, y 2 =100.

Then the sum of two small squares is 200 square meters, and * * * can raise 1200 eel fry.

To evaluate whether raising eel makes money, we should collect: the cost of eel fry, the cost of breeding, the estimated weight of mature eel, the selling price of eel fry, the mortality rate of eel fry, etc., including whether there is rent in the fish pond.