Give us some hard questions MetallicAnswered this for another user on here. Here is what I wrote:
kay so in pentane we have CH3CH2CH2CH2CH3. What came to my mind first was, will it only occur at the terminals (-CH3) or in the middle too? I concluded that it occurs in the middle too by looking at the options. First option is 2. This MAY look like it means that it only occurs at -CH3 ends, but that would mean 1 possible propogation, not 2, because it's the same thing from both sides. Since 1 isn't an option, it MUST mean that propagation can occur on ANY carbon. Now you just have to see how many unique propagations can occur. One is CH3, second is at the CH2 next to it, and third is at the middle CH2. The fourth CH2 is the same as the second and fifth CH3 is the same as the first, so UNIQUE propagation steps are only 3. Answer is B. Notice how I cleared my concept using the answer options. You're going to have to keep an open mind like this.
EDIT: the equation they gave is of methane only to show a demonstration of what happens, they are asking about PENTANE.