In my early years at school, I was taught to do subtraction sums – or take-aways, as we called them (not to be confused with takeaway restaurants, which had not yet invaded our high streets). Taking a number like 253 from, say, 967 was very simple, but when the bottom digit was greater than the one above, we were taught to borrow. Thus, 235 from 912 would prompt the following mental process: five from two you cannot do, so you borrow one at the top (making the two into 12), five from 12 leaves seven, write it down, pay back the one at the bottom turning the three into four, four from one you cannot do, and so on. This appeared to be perfectly satisfactory – you borrowed at the top, and you paid back at the bottom.
展开▼
