+0

# help

0
109
2

I was shopping for some toilet paper, I saw an advertisment for double the amount of toilet paper as a single roll. Assuming that the toilet paper is equally densely packed, and that the diameter of the core is the same at 1 unit, and the diameter of the original roll is 5 units, what is the diameter of the new roll? Jun 22, 2020

#1
-1

Double.

That means that instead of the (2.5)^2 π area, we have...

Twice!

So, 2.5^2 = 6.25

minus (0.5)^2 π = 0.25π for the area of the middle

12π+0.25π = 12.25π

sqrt(12.25) = 3.5

7 units diameter

Oops I made a mistake earlier, Corrected now

Jun 22, 2020
edited by hugomimihu  Jun 22, 2020
#2
+1

The original roll has a diameter of 5 unts   --->   radius = 2.5 units.

It's total area is  pi·2.52  =  6.25pi units2.

However, not all of that is paper; there is a hollow core of diameter 1 unit   --->   radius = 0.5 units.

The area of the core is  pi·0.52  =  0.25pi units2.

Subtracting the area of the core from the total area gives  6.25pi units2 - 0.25pi units2  =  6.00pi units2.

To get twice the amount of paper:  2 x 6.00pi units2  =  12.00pi units2.

However, we can't forget the core, so we need  12.00pi units2 + 0.25pi units2  =  12.25pi units.

Finding the square root of 12.25, we get 3.5, so the roll needs a radius of 3.5 units,

which is a diameter of 7 units.

Jun 22, 2020