Q21. Sunil has forgotten his friend’s seven-digit telephone number. He remembers the following the first three digits are either 635 or 674, the number is odd, and the number nine appears once. If Sunil were to use a trial and error process to reach his friend, what is the minimum number of trials he has to make before he can be certain to succeed?
Q21. Sunil has forgotten his friend’s seven-digit telephone number. He remembers the following the first three digits are either 635 or 674, the number is odd, and the number nine appears once. If Sunil were to use a trial and error process to reach his friend, what is the minimum number of trials he has to make before he can be certain to succeed?
The correct option is C (c) 3402
(c)
Explanation:
There are two possible cases. The number 9 comes at the end, or it comes at position 4, 5 or 6. For the first case the number would look like: 635 _ _ _ 9 / 674 _ _ _ 9. In both these cases the blanks can be occupied by any of the available 9 digits ( 0, 1, 2 .....8). Thus total possible numbers would be 2 × (9 × 9 × 9) = 1458. For the second case the number 9 can occupy any of the given position 4, 5 or 6 and there shall be an odd number at position 7. Thus the total number of ways shall be 2 [3(9 × 9 × 4)] = 1944. Hence Ans is 3402.
(c)
Explanation:
There are two possible cases. The number 9 comes at the end, or it comes at position 4, 5 or 6. For the first case the number would look like: 635 _ _ _ 9 / 674 _ _ _ 9. In both these cases the blanks can be occupied by any of the available 9 digits ( 0, 1, 2 .....8). Thus total possible numbers would be 2 × (9 × 9 × 9) = 1458. For the second case the number 9 can occupy any of the given position 4, 5 or 6 and there shall be an odd number at position 7. Thus the total number of ways shall be 2 [3(9 × 9 × 4)] = 1944. Hence Ans is 3402.
