Answer : Given: code consists of digits 3, 5, 6, 9.
To find: the largest possible number of trials necessary to obtain the correct code. The customer remembers that this 4 digit code consists of digits 3, 5, 6, 9.
So the largest possible number of trials necessary to obtain the correct code = 4! = 4 × 3 × 2 × 1 = 24