Maths Homework - again!

[Tyre Smoke]

A cuboid has a volume of 455 cm3. None of the sides are equal, but are all exact cm and all longer than 1cm.

Work out the dimensions of the cuboid.

Work out the surface area of the cuboid.




I understand the question, just can't get my head around how to work it out!

[TooLateForAName]

break 455 down into factors and go from there.

[Lynchie999]

7*5*13

should do it...

[Wiccan of Darkness]

Lynchie999 said:

7*5*13

should do it...

It does, but how do we get there?

Basically, a x b x c = 455

first, 455 is divisible by 5, that bit is easy and obvious. So divide 455 by 5 to get 91.

Now it's a lot easier to find which two numbers need to be multiplied together to get 91.

91 is not an even number so it clearly isn't 2. Or 4, 6, 8 or 10.
90 is divisible by 3. 91 is not. Same with 4, obviously isn't 5. If it isn't divisible by 3 then it certainly isn't divisible by 6.

The simple fact that 91 ends in an odd number automatically excludes division by any even number. Even before a calculator is picked up, the answer is already either 7 or 9. Just through the process of elimination, and not by randomly stabbing at the buttons on a calculator.

10x7 is 70, then + (3x7) (21) to give 91. 91 divided by 7 is 13.

Simply stabbing at numbers on a calculator won't win the points, the points will be awarded for using a logical process.

Then sketch out the cuboid.

Surface area is 2(5x7) + 2(5x13) + 2(7x13)
2x35 + 2x65 2x91

or 70 + 130 + 182

382 sq cm.

Writing down the correct numbers will only score the bare minimum of points. The examiner is trying to find out how you got there.

[TooMany2cvs]

Wiccan of Darkness said:

The simple fact that 91 ends in an odd number automatically excludes division by any even number. Even before a calculator is picked up, the answer is already either 7 or 9.

9 is instantly ruled out - all the digits in any "multiplied by 9" result add up to 9.

[Tyre Smoke]

Excellent!

Thank you everyone.

[gothatway]

TooMany2cvs said:

Wiccan of Darkness said:

The simple fact that 91 ends in an odd number automatically excludes division by any even number. Even before a calculator is picked up, the answer is already either 7 or 9.

9 is instantly ruled out - all the digits in any "multiplied by 9" result add up to 9.

9 is instantly ruled out because you have already ruled 3 out.

[Ayahuasca]

Lynchie999 said:

7*5*13

should do it...

Doesn't that have at least two equal sides?

The question says that none of the sides are equal.

[MTech535]

Should have said edges, target than sides, being a 3d shape/object.

[Lynchie999]

Ayahuasca said:

Doesn't that have at least two equal sides?

The question says that none of the sides are equal.

one of those ambiguous questions again... maybe it meant "dimensions" aren't equal ?

its not a cuboid otherwise is it ?

[Tyre Smoke]

Yes, my mistake. I meant edges.