Q. 85.0( 2 Votes )

# If ^{n}P_{r} = 840 and ^{n}C_{r} = 35, find the value of r.

Answer :

Given: ^{n}P_{r} = 840 and ^{n}C_{r} = 35

To find: r=?

We know that:

^{n}C_{r} =

and

^{n}P_{r} =

⇒ ^{n}P_{r}= ^{n}C_{r}×r!

⇒840=35×r!

⇒

⇒r!=4!

⇒r=4

Ans:r=4

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Fill in the Blanks

If ^{n}P_{r} = 840, ^{n}C_{r} = 35, then r = ______.