4. If "Cr-1 = 56, "Cr = 28 and " Cr+1 = 8, then r is equal to
(a) 8
(b) 6 (c) 5
(d) none o
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Answer is option (b), r = 6
Identity used
C(n,r)/C(n, r - 1) = (n - r + 1) / r
Answer is option (b), r = 6
Identity used
C(n,r)/C(n, r - 1) = (n - r + 1) / r
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