# destroyer QR 64

#### ddsshin

##### Full Member
10+ Year Member
there is 70% chance of rain each day for 3 days. what is prob it will rain 2 days out of 3 days?

--- can someone explain how to solve this??

#### ddsshin

##### Full Member
10+ Year Member
ok... i am not sure the difference between
n!/(n-r)! and n!/r! (n-r)! how are they used differently in QR section?

#### joonkimdds

##### Senior Member
10+ Year Member
7+ Year Member
You are asking the difference between permutation and combination.
permutation is the same award. Let's say the first 3 girls entering my room will get to have the same awards, that's permutation.

1st girl entering my room can stay 1 hr, 2nd girl can stay 30 min, 3rd girl can stay 1 min, this refers to 3 diff awards so this is combination.

#### ddsshin

##### Full Member
10+ Year Member
if 3 cards selected randomly from a deck, what is prob of 3 cards are black?

10+ Year Member

#### Streetwolf

##### Ultra Senior Member
10+ Year Member
7+ Year Member
You are asking the difference between permutation and combination.
permutation is the same award. Let's say the first 3 girls entering my room will get to have the same awards, that's permutation.

1st girl entering my room can stay 1 hr, 2nd girl can stay 30 min, 3rd girl can stay 1 min, this refers to 3 diff awards so this is combination.
You got those mixed up. Combo is order doesn't matter.

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