Adding and subtracting mentally

In Primary 2, you added and subtracted tens mentally. Today, we add and subtract mentally by using an easy number close by. Work out fifty-eight plus twenty-nine mentally by using thirty. Twenty-nine is close to thirty, and thirty is easy to add. Thirty minus twenty-nine is one. So add thirty instead. Fifty-eight plus thirty is eighty-eight. That is a jump of thirty. But we only needed to add twenty-nine, so we added one too many. So minus one. Eighty-eight minus one is eighty-seven. Now we have added just twenty-nine. The answer is eighty-seven. Fifty-eight plus twenty-nine is eighty-seven.

Work out eighty-four minus thirty-nine mentally by using forty. Try it first. Pause if you like. Thirty-nine is close to forty, and forty is easy to subtract. Forty minus thirty-nine is one. So subtract forty instead. Eighty-four minus forty is forty-four. That is a jump back of forty. But we only needed to subtract thirty-nine, so we subtracted one too many. So add one back. Forty-four plus one is forty-five. Now we have subtracted just thirty-nine. The answer is forty-five. Eighty-four minus thirty-nine is forty-five.

Work out sixty-seven plus forty-eight mentally by using fifty. Try it first. Pause if you like. Forty-eight is close to fifty, and fifty is easy to add. Fifty minus forty-eight is two. So add fifty instead. Sixty-seven plus fifty is one hundred and seventeen. That is a jump of fifty. But we only needed to add forty-eight, so we added two too many. So minus two. One hundred and seventeen minus two is one hundred and fifteen. Now we have added just forty-eight. The answer is one hundred and fifteen. Sixty-seven plus forty-eight is one hundred and fifteen. Use an easy number close by, then undo the extra.