(13) If cutting a trunk of a tree to 6 parts takes 30 minutes, how long does it take into cut it into 8 parts? (A) $30 \mathrm{~min}$ (B) $36 \mathrm{~min}$ (C) $40 \mathrm{~min}$ (D) $42 \mathrm{~min}$

Step 1 ：The problem is asking how long it would take to cut a tree trunk into 8 parts if it takes 30 minutes to cut it into 6 parts. This is a problem of proportionality.

Step 2 ：We can set up a proportion to solve this problem. The proportion would be 6 parts is to 30 minutes as 8 parts is to x minutes.

Step 3 ：Let's denote the number of parts as 'parts' and the time it takes as 'time'. For the first scenario, we have parts1 = 6 and time1 = 30. For the second scenario, we have parts2 = 8 and we need to find time2.

Step 4 ：By setting up the proportion \(\frac{parts1}{time1} = \frac{parts2}{time2}\), we can solve for time2.

Step 5 ：Substituting the given values into the equation, we get \(\frac{6}{30} = \frac{8}{time2}\).

Step 6 ：Solving this equation gives us time2 = 40.0.

Step 7 ：Final Answer: The time it would take to cut the tree trunk into 8 parts is \(\boxed{40}\) minutes.