Maths help please

[Chocovore]

DS in Y3 brought home his Maths homework Last night. There is a question, and then columns for a rounded question, estimated answer and then actual answer.

Eg. 131+48.
He has rounded this to 100 + 40, giving an estimated answer of 140 and used long addition to get the actual answer of 179. So far so good. But then we get...

89-129

How should he be doing this? He has estimated 90-100 and gets that that equals -10 but cannot calculate the actual answer. He is claiming he hasn't been taught how to calculate negative numbers....

Hmm - could it be a mistake and it's supposed to be 129 - 89? Are there other negative number questions?

There are lots of other takeaway questions but this is the only negative answer one. And by accident, this is the worksheet for the bottom group, he is normally middle group, so this to me seems a bit tricky. Hmmm....

I suspect a mistake. Probably meant to be 189 or 289.

I would either miss it out, and you write something along the lines of "Ds hasn't been shown negative numbers. Is this a mistake?"
Or assume 189.

Or do it on a calculator.

Hi Chocovore

First off your DC needs to understand that rounding is a specific process that deals with place value.

So you might be asked to round a number (which refers to units) - might be asked to round to the nearest 10 or round to the nearest tenth, etc...

Rounding follows specific rules. In the column you are rounding (so let's say it's units in the number you gave 131) - you focus on the unit digit - in this case 1 - your choices are rounding to 130 or 140 in this instance. THE RULE FOR ROUDNING: If 5 round up. So rounding 131 (1

I would assume there is an error with the negative number question, either there is a hundred unit missing, or that the question is supposed to be an addition rather than subtraction; particularly if your DS says he hasn't covered calculating with negative numbers yet.

With regards to the first question though, DD is in Year 2 and would round to the nearest 10 to give an estimated answer. In fact, she had very similar homework last week (without the negative numbers question. She would be expected to round it to 130 + 50, not 100 + 40.

89 - 129 = -(129 - 89) = -40

ie in general: number1 - number2 = - (number2 - number1)

so if the subtraction is going to give a negative result (second number bigger than first number), you swap the numbers round (new second number smaller than new first number) and take the negative of that (positive) result.

But for Y3, I think the question must be a mistake!

I would second the rounding to the nearest 10 - so 130+ 50 = 80, not the nearest 100.

Our school does do negative numbers in Yr3, but if he hasn't done them in class he won't be expected to magically understand them in homework.

Thanks all. Very helpful. I like your explanation, OutwiththeCrowd!