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I forgot my friend’s 7 digit telephone number, but I remembered that the first two digits of the number are either 25 or 53. Also the number is even and the digit 2 appears once. What is the maximum number of trials I have to make before I can contact my friend?

I forgot my friend’s 7 digit telephone number, but I remembered that the first two digits of the number are either 25 or 53. Also the number is even and the digit 2 appears once. What is the maximum number of trials I have to make before I can contact my friend?

Grade:12th pass

1 Answers

Vijay Mukati
askIITians Faculty 2590 Points
5 years ago
Dear Student,

Consider two separate cose,
Case 1: 25_ _ _ _ _ (2 is already present)
no. of possibilility = 9*9*9*9*4 (only four even numbers are possiblity in the last digit)

Case 2: 53_ _ _ _ _ Again here two different cases can occur
1. 2 is at the last digit
no. of possibilility = 9*9*9*9
2. 2 is not at the last digit
no. of possibilility = 4C1*9*9*9*4 (4C1 to choose any one place for 2, and 4 for remaing even numbers)

Add all the possibilities and get the answer.

Thanks.

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