if n =1680 and n = 70, find n and r
P C
r r

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Solution

Given : ⁿP_r = 1680, ⁿC_r = 70
Now, we know that r = ⁿP_r/ ⁿC_r= 1680/70 =24 = 4!
â‡’ r = 4
Also, ⁿP_r = n(n - 1)(n - 2)(n - 3) = 1680 = 8 Ã— 7 Ã— 6 Ã— 5 = 8(8 - 1)(8 - 2)(8 - 3)
â‡’ n = 8

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