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AIBU to disagree with the answer to this maths question?

(38 Posts)
AuntImmortelle Wed 13-May-20 10:55:32

DD answered yes to question 6 on this maths work and we've now been debating if the answer is yes or no.

The mark sheet is on the photo and says no.

I don't think there is enough info to work out the perimeter and that you can't assume the short sides are 2cm.

Argh. Maths people: what is correct??

OP’s posts: |
smeerf Wed 13-May-20 10:58:38

You can't work out the perimeter without the short sides so the answer is yes.

kathryn19801 Wed 13-May-20 10:59:14

The perimeter of this shape is the same as the perimeter of a 12x10 rectangle so there is enough information.

TeenPlusTwenties Wed 13-May-20 10:59:24

There is enough information so he is not correct.

Imagine completing the cut out rectangle.
The larger shape has perimeter 44.
But the perimeter of the larger shape and the L shape are the same.
So the l shape also has perimeter 44.

You certainly can't assume the short sides are 2cm though. They can be whatever you fancy within reason.

pickingdaisies Wed 13-May-20 11:00:29

Assuming the angle are right angles, there is enough information to work out the perimeter. Because the two shorter horizontal lines will add up to 12, and the two shorter vertical lines will add up to 10

TeenPlusTwenties Wed 13-May-20 11:01:29

Go anti-clock from Top Right.

12+10+x+(10-y)+(12-x)+y = 44 whatever the width of the two shortest sides are (in my case x and y).

AuntImmortelle Wed 13-May-20 11:03:16

@pickingdaisies that actually does make sense. Bit of a hard question when the whole lot is perimeter of irregular shapes though!!

OP’s posts: |
Keitepeheakoe Wed 13-May-20 11:03:30

There is enough information to make the assumption that this is a regular shape with opposite sides of the same length

AuntImmortelle Wed 13-May-20 11:04:24

Thank you clever people of Mumsnet! It's been buzzing in my head for an hour pissing me off!!

OP’s posts: |
2beesornot2beesthatisthehoney Wed 13-May-20 11:04:26

There is enough information . Imagine flipping the top lines making (invisible imaginary ) inner rectangle , it will join up with the shaded rectangle to make a rectangle 16x 12.

StrawberrySquash Wed 13-May-20 11:04:34

Yes, you can work it out. Short side plus long side opposite the 12cm = 12cm. Similar for 10cm.

xyzandabc Wed 13-May-20 11:05:21

I hope my diagram shows.

There is enough information.
A+C = 12cm
B+D = 10cm

You are right, you can't assume the length of the short sides but that doesn't matter when working out the perimeter.

2beesornot2beesthatisthehoney Wed 13-May-20 11:05:45

Sorry meant 10 not 16

qweryuiop Wed 13-May-20 11:07:42

@TeenPlusTwenties has a great explanation
The way I would explain to year 6 is that the two short vertical sides have to add up to 10 as they are the same length as the longer vertical side.
The two short horizontal sides have to add up to 12 for the same reason.

Can I be a nosy teacher - I know its white rose, but which year group?

AuntImmortelle Wed 13-May-20 11:13:49

Year 4!

OP’s posts: |
pickingdaisies Wed 13-May-20 11:16:59

You're welcome OP 😊

CoralFish Wed 13-May-20 11:21:56

I would say 'Yes' because the angles are not marked as right angles. That is pretty pedantic though, so I wouldn't lose sleep over being told I'm wrong.

chomalungma Wed 13-May-20 11:22:33

Love to see someone using algebra to solve it.

Always the best way grin

TeenPlusTwenties Wed 13-May-20 11:31:55

tbf I don't expect a y4 to use algebra. smile
But in the absence of pen and paper I thought it might help show the logic.

OP, the other thing that can be helpful is to pick up the paper and look at it from the side (like projections of a house). Then it becomes more obvious that the vertical lines must add to 10, and the horizontal ones must add to 12.

Being able to 'see' missing lengths is a useful skill. As is adding bits or splitting things in two or more when finding areas of odd shapes.

QueenOfToast Wed 13-May-20 11:46:24

@xyzandabc Your explanation was genius - thank you thanks

qweryuiop Wed 13-May-20 12:45:01


That's really odd! It's not even an objective until Year 5, but I've found the lesson on their website. White rose resources are fabulous, but they are pretty challenging!

smeerf Wed 13-May-20 12:54:12

I can't believe I got that wrong! I feel really silly now. I have 2 maths A levels (although they were a long time ago)

bestofme21 Thu 14-May-20 07:50:21

The answer is 44cm

bestofme21 Thu 14-May-20 07:52:30

Sorry just seen xyzandabc reply! Came to the party a little late blush

bridgetreilly Thu 14-May-20 08:48:33

It doesn't matter how long the short sides are, the perimeter will be the same.

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