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Subtraction Slip

Age 5 to 7 Challenge Level:

Well done to everyone who spotted the deliberate mistake on the video. As you may have noticed, the subtraction method needed more than one step, so you had to carefully watch the whole clip to see what went wrong towards the end of the calculation.

Krishna, from the CS Academy in India, explained what happened in the video clip:

She took away 50 and she was correct. Then she told us that she will subtract 8 but she subtracted 4 from 12. 

Jasmine, from Meavy Church of England Primary, also spotted the mistake as well as suggesting a way to make the calculation correct:

I did a number line and the mistake was that 12 - 8 is not 8 , it equals 4. If you had 12 sweets and you took away 8, there would be 4 sweets left. So the answer would be 4. If you had 16 sweets and took away 8, it would equal 8.

Thank you both.

Aria, from St Michael's School, in Japan, sent in this number line showing the correct working out and answer:

As many of you realised, there's several ways that you can work out 62 - 58. In the video, we saw a number line being used to try to find the answer. Krishna suggested a different way to calculate 62 - 58. See if you can follow this method too:

STEP ONE

62 - 2 = 60

STEP TWO

60 - 2 - 58

STEP THREE

2 + 2 = 4

So 62 - 58 = 4

Krishna preferred this counting backwards approach than using a number line:

The reason I changed the method was that it takes three easy steps to find out how much greater 62 is than 58.

Do you agree with Krishna?

Aria shared this different method:

58 + 2 + 2 = 62

So 62 - 58 = 4

It has fewer steps so you are less likely to get mixed up and make a mistake.

What do you notice about Aria's and Krishna's methods? What's the same about them? What's different?

As we saw in the video clip, it's very easy to make a mistake when we calculate. If you checked your answer to 62 - 58, how did you do it? I wonder if you could have checked it another way too?