If 20 J of energy is trapped at producer level, then how much energy will be available to peacock as food in the following chain?  

plant → mice → snake → peacock

Energy trapped at producer level, plant = 20 J

Energy trapped by mice = 20 x 10% = 2 J

Energy trapped by snake = 2 x 10% = 0.2 J (available to peacock)

Energy trapped by peacock = 0.2 x 10% = 0.02 J

Energy available to peacock is the energy trapped by the snake.