Question

If n p r =840 and n c r =35 , then find r.

Answer 1

n

C

r

n

P

r

=r!=

35

840

=24⇒r=4

n

P

r

=

(n−r)!

n!

=840

⇒n×(n−1)×(n−2)×(n−3)=840

⇒840=4×5×6×7

⇒n=7

Answer 2

Answer:

r = 4 is correct answer for this question.