We are not used to do quick mental calculations, since we have several electronic devices which can do this for us. Electronic devices are very useful for everyday life, but indeed they made our minds lazy. Sometimes, we haven't got an electronic calculator, so we need to do calculations mentally. Brain training is a key strategy to live above our targets. Computing ability is not in our nature, but thanks to a correct training, it can be developed. Look for the right answer by making a quick mental calculation, which has its score in one of the suggested options.