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Assuming that the question means that her force is applied 20 degrees below the vertical,  resolve this force. Use work done = force x distance moved in the direction of the force to calculate work done in pushing the rice. Then express this as a percentage of the total energy in. I make the answer 0.008%.

Draw a diagram! It's not clear from the way the question is written, whether the force she is exerting makes a 20* angle with the horizontal but I think that is what is meant. You need to work out the proportion of the force applied that is working horizontally - using Pythagorus. Then you can work out the "work done" and divide that by the total input energy to get a proportion of the food energy used in this exercise. Hope that helps. 