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A customer forgets a four digit code for...

A customer forgets a four digit code for an Automatic Teler Machine (ATM) in a bank. However, he remembers that this code consists of digits 2, 3, 6 and 9. Find the largest possible number of trials necessary to obtain the correct code.

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The correct Answer is:
24
1st, 2nd, 3rd, 4th digits may be chosen in 4 ways, 3 ways, 2 ways and 1 way respectively.
Number of largest possible trials `=(4xx3xx2xx1)=24.`
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