Given n ! = n ( n - 1 ) ( n - 2 ) ............ ´ 4 ´ 3 ´ 2 ´ 1
\ ( n + 1 ) ! = ( n + 1 ) ( n ) ( n - 1 ) ( n - 2 ) ( n - 3 ) ..... ´ 3 ´ 2 ´ 1
( n - 1 ) ! = ( n - 1 ) ( n - 2 ) ( n - 3 ) ......... ´ 3 ´ 2 ´ 1
From above
( n + 1 ) ! = ( n + 1 ) ( n ) ( n - 1 ) ! OR
\ n ( n + 1 ) = 20
But n and n + 1 are consecutive integers whose product is 20.
Hence n = 4 and n + 1 = 5.
Hence E