Question

If npr=1680 and ncr =70 then find the value of n and r

Answer 1

Step-by-step explanation:

npr = r! × ncr

1680 = r! ×70

r! = 1680/70 = 24 = 4×3×2×1 =4!

r = 4

npr = n!/ (n-r)! = 1680

=n!/(n-4)! =1680

= n*n-1*n-2*n-3*(n-4)!/(n-4)! = 1680

= n*n-1*n-2*n-3 =1680

= 10*4*42=10×4×2×21 = 5×7×8×6

n = 8