Question

IF 28C2r: 24C2r-4= 225 : 11, then the value of r is
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Answer 1

In the problem given it is rin place of 2r..

28Cr : 24 C(n-4) = 225 : 11

Answer = r = 11

\frac{28*27*26...(28-n+1)}{n(n-1)(n-2)(n-3)...2.1} : \frac{(n-4)!}{24*23*22...(24-n+4+1)} = 225 : 11\\\\\frac{28*27*26*25}{n(n-1)(n-2)(n-3)} : 1 = 225 : 11\\\\\frac{28*3*26}{n(n-1)(n-2)(n-3)}=\frac{1}{11}\\\\n(n-1)(n-2)(n-3)=28*3*26*11=11*12*13*14\\\\n=11