JAMB 2012 · UTME · Q47

Evaluate ${}^{n + 1}C_{n - 2}$ if $n = 15$ .

Worked solution (try it first)

With $n = 15$ , $^{n + 1}C_{n - 2} = {}^{16}C_{13}$ .
Choosing 13 is the same as leaving out 3: $^{16}C_{13} = {}^{16}C_3$
$= \dfrac{16 \times 15 \times 14}{3 \times 2 \times 1}$ .
That is $\frac{3360}{6} = 560$ , option D.