me test series

Question

Consider a bit-string of length 10 containing only 0 and 1. The number of string contain exactly 3 0’s or exactly 31’s are ________

Answer 1

The number of string containing exactly 3 0’s or exactly 3 1’s in a group of 10 bit string are :

number of string containing exactly three 0’s $+$ number of string containing exactly three 1’s are

$-$ number of string containing exactly three 1’s and exactly three 0's

$= 10_{c_3}+10_{c_3}-0 \\ =2* 10_{c_3} \\ =2*120 \\ =240$ 

Comments

how to do if want to use both permutation and combination

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how to do if want to use both permutation and combination

Didn`t get you ? 

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yep, this is correct answer!