Question:
A customer forgets a four-digit code for an automated teller machine (ATM) in a bank. However, he remembers that this code consists of digits 3, 5, 6, 9. Find the largest possible number of trials necessary to obtain the correct code.
Solution:
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 $=4 !=4 \times$ $3 \times 2 \times 1=24$