If the coefficients of $r^{t h}$ term and $(r + 4)^{t h}$ term are equal in the expansion of $(1 + x)^{20}$ , then the value of r will be

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Solution

The correct option is C
9

$^{20} C_{r - 1}$ = $^{20} C_{r + 3}$ ⇒ 20 - r + 1 = r + 3 ⇒ r = 9.

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