the expression 400!/398! is equivalent to

400!  =  400 x 399 x 398!

So, 400! / 398!  =  400 x 399  = 159 600

Note that (n+2)! is greater than n! by (n+2)(n+1)......so using 398 = n, we have

(398+2)*(398 + 1) = 400*399 = 159,600