EEG data classification using energy

I have two cognitive tasks (relaxing and performing math operations), and i have 40 cases(20 for each cognitive task). I need to extract features from the EEG data.I should use these datas energies in the well-known EEG frequency bands: delta (0-4 Hz), theta (4- 8 Hz), alpha (8-12 Hz), beta (12-30 Hz) and gamma (>30 Hz). Pls help me