3rd gr. math help, please

[LeslieG]

Here is the question: "A cube has a base of 40 square feet and a height of 70 feet. What is the volume of the cube? Show your work."

Thanks.

 

[BCDisneyFanatic]

Here is the question: "A cube has a base of 40 square feet and a height of 70 feet. What is the volume of the cube? Show your work."

Thanks.

The volume of a cube is equal to its base (length times width), multiplied by its height. It's base is 40 sq. ft, and its height is 70ft. Therefore, it's volume is:

V = base x height
= 40 sq. ft x 70 ft.
= 2800 cubic feet

 

[Disney1fan2002]

v= b x h

 

[BCDisneyFanatic]

Oh...I was also assuming that they were talking about a 'box' or rectangular prism rather than a 'cube' since a 'cube' would indicate that each of its dimensions were equal (length, width, height), but since the base is 40 square feet, there is no way its length and height are each 70 ft (the height of the object).

 

[december]

Yep, somebody flubbed the question!

 

[FireDancer]

Yep, somebody flubbed the question!

Exactly. As far as I can remember a cube has equal edge lengths. If the 'cube' in question has equal angels it is either a cuboid (all right angels) or a parallelepiped (non right angels). With the information given you can't figure the volume of any of these. Since the formula for figuring the volume of a parallelepiped is far too advanced for a 3rd grader I can only assume the teacher meant cuboid. Unfortunately, you can't deduce the length of the 3rd side of the cuboid based on the other two without either the volume or surface area.

Sounds like a teacher needs a refresher course.

 

[BCDisneyFanatic]

Exactly. As far as I can remember a cube has equal edge lengths. If the 'cube' in question has equal angels it is either a cuboid (all right angels) or a parallelepiped (non right angels). With the information given you can't figure the volume of any of these. Since the formula for figuring the volume of a parallelepiped is far too advanced for a 3rd grader I can only assume the teacher meant cuboid. Unfortunately, you can't deduce the length of the 3rd side of the cuboid based on the other two without either the volume or surface area.

Sounds like a teacher needs a refresher course.

Yes, in third grade they learn the formula for 'cuboids' or rectangular prisms (or boxes to the average 9-year-old). However, other than calling it a cube, the rest of the information is just fine. You don't need to deduce anything (other than the volume)...the information is given. The 'base' (or cross-sectional area of the prism) is 40 square feet. This represents two of the three dimensions of our prism. Knowing the area of the base, we can multiply that by our third dimension (the height) to determine the volume of the prism.

Perhaps you can attend that refresher course with the OP's teacher?

 

[FireDancer]

Yes, in third grade they learn the formula for 'cuboids' or rectangular prisms (or boxes to the average 9-year-old). However, other than calling it a cube, the rest of the information is just fine. You don't need to deduce anything (other than the volume)...the information is given. The 'base' (or cross-sectional area of the prism) is 40 square feet. This represents two of the three dimensions of our prism. Knowing the area of the base, we can multiply that by our third dimension (the height) to determine the volume of the prism.

Perhaps you can attend that refresher course with the OP's teacher?

Ah, I missed the square foot part and thought 40 ft. was the length of the base.

The question needs to be reworded to either ask for the area of a cuboid or give the dimensions of an actual cube. A cube with a height of 70 feet could not have a base of 40 square feet.

 

[LeslieG]

I thought it was just me who thought the question was missing something. But the bottom line is we answered it 2,800. Hope that's right.

 

[crashbb]

Yes, in third grade they learn the formula for 'cuboids' or rectangular prisms (or boxes to the average 9-year-old). However, other than calling it a cube, the rest of the information is just fine. You don't need to deduce anything (other than the volume)...the information is given. The 'base' (or cross-sectional area of the prism) is 40 square feet. This represents two of the three dimensions of our prism. Knowing the area of the base, we can multiply that by our third dimension (the height) to determine the volume of the prism.

Perhaps you can attend that refresher course with the OP's teacher?

Will they also teach that there are no angels in a cube?

I would go with 2,800.

 

[BCDisneyFanatic]

Will they also teach that there are no angels in a cube?

No, that will be addressed in the remedial language arts course.

 

[si-am]

The question is, why are all of you doing this child's homework in the first place?

 

[Magpie]

Here is the question: "A cube has a base of 40 square feet and a height of 70 feet. What is the volume of the cube? Show your work."

Thanks.

You know, after you've solved this one, you might consider taking some sugar cubes and showing your child why the formula works.

For instance, if you made a base of four sugar cubes (representing the 40 square feet), and then stacked them 7 high (70 feet), you can see that you've got 28 sugar cubes - or 2800 feet.

You can also show how a base of 40 square feet can be arranged in any fashion - a square, a triangle or even L - and it doesn't change the volume.

Once you can understand it in a concrete way, you won't forget that volume is length times width times height.

 

[cocodog]

I think this is a trick question. A cube would have all 3 dimensions the same.