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Question
If 15Cr : 15Cr − 1 = 11 : 5, find r.
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Solution
Given:
15Cr : 15Cr − 1 = 11 : 5
We have,
\[\frac{{}^{15} C_r}{{}^{15} C_{r - 1}} = \frac{11}{5}\]
\[\Rightarrow 75 - 5r + 5 = 11r\]
\[ \Rightarrow 16r = 80\]
\[ \Rightarrow r = 5\]
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