Surface area of box when laid flat

11 Plus Maths – Preparation and Information

Moderators: Section Moderators, Forum Moderators

Post Reply
11 Plus Platform - Online Practice Makes Perfect - Try Now
compu
Posts: 7
Joined: Fri Sep 03, 2021 11:10 pm

Surface area of box when laid flat

Post by compu »

Hi guys,

How do we find the surface area of a box when laid flat.

L=30,
W=15
H=4

Answer is 810.

Is it 2hw+2lw+lh?
KaB£H1s3
Posts: 167
Joined: Sun Oct 04, 2020 2:15 pm
Location: Trafford

Re: Surface area of box when laid flat

Post by KaB£H1s3 »

2x lh at the end assuming the box has 6 faces
compu
Posts: 7
Joined: Fri Sep 03, 2021 11:10 pm

Re: Surface area of box when laid flat

Post by compu »

KaB£H1s3 wrote:2x lh at the end assuming the box has 6 faces
Sorry, didn't get what you mean?
ToadMum
Posts: 11945
Joined: Wed Jan 18, 2012 12:41 pm
Location: Essex

Re: Surface area of box when laid flat

Post by ToadMum »

compu wrote:
KaB£H1s3 wrote:2x lh at the end assuming the box has 6 faces
Sorry, didn't get what you mean?
By only adding 'length x height' once, you assume (or know) that the box has one open face. Do you have a diagram of the box to go with the measurements? Otherwise, how do you know that it is one of the 'lh' faces that is open and not a 'wh' or 'lw' one?

Checking back from what you know to be the right answer and using your formula:

(L=30, W=15, H=4)

2hw+2lw+lh

hw =60, x2 = 120
lw = 450, x2 = 900
lh = 120

Total = 1140

So it isn't an open box with one of the long sides open (like something you slot e.g. a book into), which your formula describes. Nor is it a box with 6 faces.

But if you work out the surface area of one with 6 faces, by adding another 'lh', you get 1260 square units (?cm).

You can then take off the area of each type of face in turn, until you get a total which matches one of the available answers.

(I am assuming that this is a multiple choice question, because otherwise either you would have to assume a 'closed box', in which case the answer would have to be 1260, or you have a diagram of the box showing which face is open, therefore you know which face only to include once. And if the answer is 810, the open face has to be one of the 'lw' ones, not one of the 'lh'. 1260 - 810 = 450).
Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.Groucho Marx
Blitz
Posts: 874
Joined: Wed Aug 25, 2010 3:58 pm

Re: Surface area of box when laid flat

Post by Blitz »

I use the mantra 'If in doubt, draw it out.'

By drawing out the 30 x 15 x 4 box, you will notice there are:

2 faces of 30 x 15

2 faces of 30 x 4

2 faces of 15 x 4

So 30x15x2 = 450
30x4x2 = 240
15x4x2 =120

Total surface area = 810 cm squared
compu
Posts: 7
Joined: Fri Sep 03, 2021 11:10 pm

Re: Surface area of box when laid flat

Post by compu »

Hi,

Thanks for replying.

I re-read the question and found out that I missed one of the key information provided in question. (The question says, OPEN TOP box) and the drawing shows which side is top.

Now I get the answer.

Thanks once again.

(The question is from Bond Maths Assessment papers Book 2. 11+ to 12+ years)
Post Reply
11 Plus Platform - Online Practice Makes Perfect - Try Now