Answer:
She needs 301.8 in² of tissue paper.
Explanation:
In order to calculate the amount of tissue paper she needs to make 21 hats, we need to find the area of each hat and multiply it by the total number of hats she wants to make.
Since the Vietnamese conical hats are hollow and don't have a base, we need to find the area of the it's lateral walls, this is given by the following expression:
lateral area = pi*r*l
r = d/2 = 16/2 = 8 in
We were given the height, but we need the slant height, therefore:
l = sqrt(r² + h²) = sqrt(8² + 9²) = sqrt(64 + 81) = sqrt(145) = 12.041 in
Then the lateral area is:
lateral area = pi*8*12.0141 = 301.8 in²
She needs 301.8 in² of tissue paper.