Math Problem: Combinations?

[匿名]

2007-11-28 11:35 am

Can you help me with this?

A box contains 7 red balls, 5 white balls, and 3 green balls. in how many ways can we select 3 balls such that -

a. they are all red?
b. they are all white?
c. they are all green?
d. they are of different colors?
e. two are red and one is white?
f. exactly 2 are white?
g. exactly 1 is green?
h. none is green?

Thank you so much!

回答 (1)

pyrodude1031

2007-11-28 12:41 pm

✔ 最佳答案

a. 7C3 because .. choosing 3 red balls from the 7 you have
b. 5C3 because ... choosing 3 white balls from the 5 you have
c. 3C3 = 1 because choosing 3 from the 3 you only have hence 1 way to do tha
d. (7C1)(5C1)(3C1) = 105 , imagine you are going from town A to town D passing B and C, and there are a number of ways from each town to the next... hence 7 * 5*3
e. 7C2 * 5C1
f. 5C2 * (7+3)C1, you must take into acc of the extra ball you must obtain to get three balls
g. 3C1 * (7+5)C2
h. (7+5)C3

參考: Myself

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