Question

if nPr=840, nCr=35,find n.

Answer 1

Hi ,

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We know that ,

1 ) nPr. = n!/( n - r )!

2 ) nCr = n!/ [ ( n - r )! r ! ]

3 ) r ! = ( nPr )/ nCr

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It is given that ,

nPr = 840 ---( 1 )

nCr = 35 ---( 2 )

r ! = ( nPr )/ ( nCr )

= 840/35

= 24

r ! = 4 × 3 × 2 × 1

r ! = 4!
Therefore,

r = 4

Substitute r value in equation ( 1 ),

We get

n!/ ( n - 4 ) ! = 840

n( n - 1 )( n - 2 )( n - 3 ) = 840

n( n - 1 )( n - 2 )( n - 3 ) = 7× 6 × 5 × 4

Therefore ,

n = 7

I hope this helps you.

: )

Answer 2

Answer:

n = 7 is correct answer.