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C(10,4)*C(6,6) = (10!/(4!*6!))*(6!/(0!*6!)) = (7*8*9*10)/(1*2*3*4) * 1 = 5040/24 = 210 | C(10,4)*C(6,6) = (10!/(4!*6!))*(6!/(0!*6!)) = (7*8*9*10)/(1*2*3*4) * 1 = 5040/24 = 210 | ||
| − | + | [[Midterm Practice Questions]] | |
[[Category:MA375Spring2009Walther]] | [[Category:MA375Spring2009Walther]] | ||
Revision as of 17:32, 9 March 2009
1. How many bitstrings of length 10 have exactly 4 zeros?
We know that every bitstring has exactly 4 zeros, meaning that it also has exactly 6 ones. So, I think that the solution to this problem would be to find the number of ways that 4 zeros can be placed into 10 spaces and then 6 ones placed into the remaining 6 spaces.
So, placing 4 indistinguishable objects (zeros) into 10 spaces:
C(10,4)
And, placing 6 indistinguishable objects (ones) into the remaining 6 spaces:
C(6,6)
Gives us
C(10,4)*C(6,6) = (10!/(4!*6!))*(6!/(0!*6!)) = (7*8*9*10)/(1*2*3*4) * 1 = 5040/24 = 210
