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Q7 of 68 Page 330

If ¹⁵C_r : ¹⁵C_r–1 = 11 : 5, find r.

Given: ¹⁵C_r : ¹⁵C_r–1 = 11 : 5

To find: r=?

¹⁵C_r : ¹⁵C_r–1 = 11 : 5

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⇒5×(16-r)=11r

⇒80-5r=11r

⇒16r=80

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⇒r=5

Ans:r=5

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