Question

If ${}^{15}{C}_{3r}{=}^{15}{C}_{r+3},$ then r is equal to

• A. 5
• B. 4
• C. 3
• D. 2

Answer 1

The correct option is C

3

$3r+r+3=15$

$[\because {}^n{C}_x={}^n{C}_y\Rightarrow n=x+yorx=y]$

$\Rightarrow 4r+3=15$

$\Rightarrow 4r=12$

$\Rightarrow r=3$