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How many four digit numbers have the property that their digits taken from left to right form an Arithmetic or a Geometric Progression? (a) 15 (b) 21 (c) 20 (d) 23 |
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Answer» Correct option (d) 23 Explanation: The numbers forming an AP would be: 1234, 1357, 2345, 2468, 3210, 3456, 3579, 4321, 4567, 5432, 5678, 6543, 6420, 6789, 7654, 7531, 8765, 8642, 9876, 9753, 9630. A total of 21 numbers. If we count the GPs we get: 1248, 8421-a total of 2 numbers. Hence, we have a total of 23, 4-digit numbers why the digits are either APs or GPs. |
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