Ithink of a number double it add 15 my number is 21 what number was i thinking about

Hello from MrBillDoesMath!

3

Discussion:

Let the number be "n",

2n + 15  = 21          =>  subtract 15 from both sides

2n = 21 - 15 = 6     =>   divide both sides by 2

n = 6/2 = 3

Check: Start with 3. Double it (gives 6), add 15 to it ( 6 + 15)  which gives 21/

Thank you,

MrB

The question is incomplete. The complete question is :

A sphere and a cylinder have the same radius and height. The volume of the cylinder is 54 meters cubed. Amie found the volume of the sphere. A sphere with height h and radius r. A cylinder with height h and radius r. Her work is shown below. V = two-thirds + 54. V = two-thirds + StartFraction 162 Over 3 EndFraction. V = StartFraction 164 Over 3 EndFraction meter cubed. What is Amie's error?

Solution :

Amie should have multiplied 54 by two-thirds.

Given that sphere and the cylinder have the same radius as well as same height. The volume of the cylinder is 54 inch cube.

Volume of the sphere =      .......... (i)

Volume of cylinder =   ....................(ii)

Therefore, the ratio of sphere volume to cylinder volume is,

  ......(iii)

Divide both the sides by , we get

         ..........(iv)

We know that the height of the sphere = diameter of the sphere

The diameter of the sphere is D = 2r

Also the sphere height = cylinder height

So, the height of the cylinder = 2r

Therefore, substituting the height of the cylinder as 2r that is represented as h in equation (iv) is given by :

       .............(v)

Now dividing both the sides by 2r, we get

  ..................(vi)

Thus for equation (vi), we see, sphere volume =  of the cylinder volume

∴  sphere volume =

                              = 36 meter cube

Thus, Amie should have multiplied 54 by .