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If \ ^20 Cr=\ ^20 C(r-10) ,\ t h e n\ \ ...

If `\ ^20 C_r=\ ^20 C_(r-10) ,\ t h e n\ \ ^18 C_r` is equal to a. 4896 b. 816 c. 1632 d. none of these

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As
`n_(C_x)=n_(C_y)`
So,
`r-10+r=20`
`2r-10=20`
`2r=30`
`r=15`
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