in food chain 20J is available with the hawks how much energy is available with the plants?
plants-->rats-->snaks-->hawks

I think the energy with plants will be 20000 Joules by applying the 10% law..

pants contains 20kj = 20,000j

20,000 J due to 10% law

and if 20J is available to the hawk not with the hawk. then how much energy is available with the plants?
 

By 10% law,  Plants- 20000J, Rats- 2000J, Snakes- 200J & Hawk- 20J

If 20J is available to the hawk that means it is 10% of the energy of the snakes.
Now let the energy of plants be x Joule
now
energy of rats = 10% of energy of plants
= (10× x) ÷ 100
energy of snakes = 10% of energy of rats
= (10×10× x)÷100×100
= x÷100
so
energy of hawk = 10% of energy of snakes
20J = (10×x)÷(100×100)
20J = x÷1000
20J × 1000 = x

therefore
energy of plants = 20000J