Human neuroimaging studies have shown that intraparietal sulcus (IPS) could have a central amodal representation of quantity. However, experts’ adding large numbers with many digits in very short time may not be simply in this way. An expert who has taken a long period of abacus training and been awarded many champions performed several adding tasks (with 8-digits and 4digits numbers either presented by a LED screen or played by speakers) in this study. Functional magnetic resonance imaging (fMRI) showed that in both visual and auditory adding tasks, bilateral superior parietal lobule (brodmann area 7) and middle frontal gyrus (brodmann area 40) were activated, compared with control tasks. Electroencephalography (EEG) topography on scalp also showed bilateral parietal and frontal activity at around 400ms after visual stimulus presented. Phase synchrony analysis indicated that the parietal and frontal brain areas collaborated with each other in alpha band and this connection was stronger in adding tasks than control tasks. This could be seen in both visual and auditory tasks. In conclusion, visual and auditory information converge to the same central system involving parietal and frontal loop which execute calculation process.