Problems in PNC

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1.The no. of postive integral sol. for abc=42 is a)17 b)27 c)21 d)3!*42 2\. for sigma K=m to n kCr equals: (a) (n+1)C(r-1) (b)(n+1)C(r+1) - mCr (c) (n+1)C(r+1) - mC(r+1) (d)(n+1)C(r+1)+ mC(r+1) 3.In a certain test are n q...
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1.The no. of postive integral sol. for abc=42 is
a)17
b)27
c)21
d)3!*42

2. for sigma K=m to n kCr equals:
(a) (n+1)C(r-1)
(b)(n+1)C(r+1) - mCr
(c) (n+1)C(r+1) - mC(r+1)
(d)(n+1)C(r+1)+ mC(r+1)

3.In a certain test are n question. In this test 2^(n-k) students gave wrong answers to atleast k questions, where k=1,2,3...n . If total no. of wrong answers given is 2047, then n is equal to:
(a)10
(b)11
(c)12
(d)13

though answers are given in book but they are not useful I need explnation for each question. Thank in adv. 😃

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