Check the picture below.
since chords NQ and MP cross the center of the circle at R, that means that those two chords are diametrical chords and the angles made by both are vertical angles and thus twin angles, namely both are 18° as you see in the picture, so the angle NMP in magenta is really 162° + 18° + 18° = 198°, and we know the radius NR is 8.
![\textit{arc's length}\\\\ s=\cfrac{r\pi \theta }{180}~~ \begin{cases} r=radius\\ \theta =\stackrel{degrees}{angle}\\[-0.5em] \hrulefill\\ r=8\\ \theta =198 \end{cases}\implies s=\cfrac{(8)\pi (198)}{180}\implies s\approx 27.6](https://tex.z-dn.net/?f=%5Ctextit%7Barc%27s%20length%7D%5C%5C%5C%5C%20s%3D%5Ccfrac%7Br%5Cpi%20%5Ctheta%20%7D%7B180%7D~~%20%5Cbegin%7Bcases%7D%20r%3Dradius%5C%5C%20%5Ctheta%20%3D%5Cstackrel%7Bdegrees%7D%7Bangle%7D%5C%5C%5B-0.5em%5D%20%5Chrulefill%5C%5C%20r%3D8%5C%5C%20%5Ctheta%20%3D198%20%5Cend%7Bcases%7D%5Cimplies%20s%3D%5Ccfrac%7B%288%29%5Cpi%20%28198%29%7D%7B180%7D%5Cimplies%20s%5Capprox%2027.6)

Put the values of c = 6 and d = 7 to the expression:

The porosity of the gravel sample is 28.75%
We have to determine.
What is the porosity of the gravel sample?
<h3>The porosity of the gravel sample;</h3>
Porosity is the term which is defined as the volume of voids present in the soil to the total volume of the soil.
The formula used to calculate the Porosity is;

A sponge is a case of a permeable fabric because it incorporates a huge number of purge spaces compared to its volume.
Porosity is the property of a question that communicates the entire volume of purge or pore space within the material.
Hence, the porosity of the gravel sample is 28.75%
Learn more about "porosity" :
brainly.com/question/2593663