Answer
$_{40}C_{40}=1$
Work Step by Step
RECALL:
$_nC_r = \dfrac{n!}{r!(n-r)!}$
Use this definition to have:
$\require{cancel}
_{40}C_{40}=\dfrac{40!}{40!(40-40)!}
\\_{40}C_{40}=\dfrac{40!}{40!(0!)}
\\_{40}C_{40} = \dfrac{40!}{40!(1)}
\\_{40}C_{40} = \dfrac{\cancel{40!}}{\cancel{40!}}
\\_{40}C_{40}=1$
