12.1.1 - 12.1.4 Reading

1. Difficult - I am blanking on the explanation the book gives on permutations. I understand logically why we remove one 'outcome' for each 'place' when we calculate but the blue box I do not understand as well as the simplification to
P(n,k) = n! / (n-k)!
I don't understand where the n(n-1)(n-2) x (n-k+1) is coming from.

2. Reflective - Haha, this section is giving me bad flashbacks of high school trig when I didn't understand any of the problems that use/d a combination of counting techniques. We also did very similar problems that dealt with the same items: cards, license plates, colored balls in a bag, etc.
I think I need to get this under my belt because it seems to be simple for everyone else and because if I decide not to do medicine and pursue ecology or something I will need to know these probability/statistics techniques very well in order to do research (i.e. maybe something like how many different types of fly can exist with the following traits: regular wings v. vestigial wings, red eyes v. white eyes, etc. We did something similar in Biology AP with Drosophilia).