Maths Olympiad Prep

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Number theory Difficulty 3.8 AMC 10/12 Find the answer Slovenia

How many two-digit numbers are there, such that the predecessor and the successor are a prime and a perfect square, not necessarily in this order?

Pick one

Solution

Perfect squares that are also predecessors or successors of two-digit numbers are 99, 1616, 2525, 3636, 4949, 6464, 8181 and 100100. Two-digit numbers that have a perfect square as their successors or predecessors are therefore 1010, 1515, 1717, 2424, 2626, 3535, 3737, 4848, 5050, 6363, 6565, 8080, 8282 and 9999. But a two-digit number that is preceded or succeeded by a prime must be even, so the only options are 1010, 2424, 2626, 4848, 5050, 8080 and 8282. Out of these, the only ones with a prime neighbour are 1010, 2424, 4848, 8080 and 8282. All five of these satisfy the condition. Thus, the correct answer is 55.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.