The 'Britain´s Got Talent' of south korea, 'Star King' introduced the expert of mental arithmetic. She did quick mental arithmetic to calculate large integer numbers in only a few seconds. For example, 258,785,412,354 + 125,465,871,235 + 954,125,487,965 - 845,123,658,792 * 542,136,587,212 ... = ?
I was very impressed. According to She, "I did calculate them with an imaginary abacus".
OK, The abacus-based mental calculation is the recipe. Generally, we learned to calculate multi-digit numbers from right to left, one digit at a time. It's a carry/serial calculation. But Her (abacus) algorithm is little different from us. It's the shift-up/parallel calculation.