**How to Count - Number of Ways (경우의 수)**

Sum Rule : when the events cannot happen at the same timeProduct Rule : when the events happen in conjunction AND the number of leaves for each branch of the problem space tree is the sameHow to classify the problem space into events/sets : Rule 1) Do it exhaustivelyRule 2) Avoid over-counting!ex) There are N kids running around the field. How will you count them?Let's say we use the groups "Boy..

**How to Count (The Fencepost Problem) - Off by One Errors!**

1 ~ n : n itemsThis is easy. You only need to read the last item's number to get the number of items.8 ~ 11 : 4 numbers11 - 8 = 3 numbers? WRONG!!! Why? Subtraction = the number of spans between numbers, NOT a count of how many thingsA span = distance or some quantity measure.8am ~ 11am has 3 units (hours) of time8cm ~ 11cm has 3 units (cm) of distance 8 ~ 11 floors means we are counting the num..

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