AIBU to disagree with the answer to this maths question?

[AuntImmortelle]

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??

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

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

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.

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

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).

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

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

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

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.

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

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.

Sorry meant 10 not 16

@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?

Year 4!

You're welcome OP 😊

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.

Love to see someone using algebra to solve it.

Always the best way

tbf I don't expect a y4 to use algebra.
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.

@xyzandabc Your explanation was genius - thank you

@AuntImmortelle

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!

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)

The answer is 44cm

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

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

Assuming the angles are right angles then it is possible.