1.

If 20Cr = 20Cr+4, then rC3 is equal to :A. 54 B. 56 C. 58 D. none of these

Answer»

Option : (B)

r + r + 4 = 20 

(nCx = nCy ⇒ n = x + y or x = y ) 

2r + 4 = 20 

2r = 16 

r = 8 

Now,

rC3 = 8C3

8C3 = \(\frac{8!}{3!5!}\)

= \(\frac{8\times 7\times 6}{3\times 2\times 1}\)

= 56


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