Cream margarine and sugar well. Add egg and egg whites and beat well. Combine flour, baking powder, baking soda and salt. Add to sugar mixture and beat until mixed. Add banans and yogurt and blend well. Put in a 13 inchx9 inch greased baking pan and bake at 180ºC or 40-45 mins or until a cake tester comes out clean.Cool on a rack.