If ( ^ 20 C_ r = ^ 20 C_ r-10 ) then ( ^ 18 C_ r ) is equal to

If ({ }^{20}

\mathrm{C}_{\mathrm{r}}={ }^{20} \mathrm{C}_{\mathrm{r}-10}

) then ({ }^{18}

\mathrm{C}_{\mathrm{r}}

) is equal to

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B. 816

({ }^{20}

\mathrm{C}_{\mathrm{r}}={ }^{20} \mathrm{C}_{\mathrm{r}-10} \Rightarrow \mathrm{r}+(\mathrm{r}-

10)=20 \Rightarrow

\mathrm{r}=

15) (\therefore \quad{ }^{18}

\mathrm{C}_{\mathrm{r}}={ }^{18} \mathrm{C}_{15}={ }^{18} \m

mmathrm{C}}_{3}=\frac{18 \cdot17 \cdot16}{1 \cdot2 \cdot3}=816)