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If .^(n)P(r) = 720 and .^(n)C(r) = 120, ...

If `.^(n)P_(r) = 720` and `.^(n)C_(r) = 120`, then find r.
`= (5 xx 4 xx 3)/(3 xx 2 xx1) xx (4xx 3 xx 2)/(3 xx 2xx1)`
`= 10 xx 4 = 40`.

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The correct Answer is:
`r = 3`
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NAGEEN PRAKASHAN-PERMUTATION AND COMBINATION -Exercise F
  1. If .^(16)C(r) = .^(16)C(r+6), then find .^(5)C(r).

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  2. Determine n if (i) ^2n C2:^n C2=12 :1 (ii) ^2n C3:^n C3=11 :1

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  3. Determine n if (i) ^2n C2:^n C2=12 :1 (ii) ^2n C3:^n C3=11 :1

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  4. Determine n if (i) ^2n C2:^n C2=12 :1 (ii) ^2n C3:^n C3=11 :1

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  5. If .^(15)C(r): .^(15)C(r-1) = 1:5, then find r.

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  6. If .^(n-1)P3 :^(n+1)P3 = 5 : 12, find n.

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  7. If .^(n)P(r) = 720 and .^(n)C(r) = 120, then find r. = (5 xx 4 xx 3...

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  8. If ^(n+1)C(r+1): ^nCr: ^(n-1)C(r-1)=11:6:2 find the values of n and r.

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  9. If .^(n)C(4),.^(n)C(5), .^(n)C(6) are in A.P., then find the value of ...

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  10. If alpha=\ \ ^m C2,\ then find the value of \ ^(alpha)C2dot

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  14. In how many ways can 11 players be selected from 14 players if (i) a...

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