Here's how we can find the number of 3-digit numbers with exactly 9 factors:
Factorization: A number with 9 factors must be a perfect square of a number with 5 factors (since the factors come in pairs). For example, 256 has 9 factors because it's the square of 16, which has 5 factors (1, 2, 4, 8, 16).
Perfect Squares: To find 3-digit perfect squares, we can start by listing the perfect squares from 100 to 999: 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576, 625, 676, 729, 784, 841, 900.
Counting Factors: Now, we need to check which of these perfect squares have a number with 5 factors. We can do this by prime factorizing each number and counting the number of factors using the formula: (exponent of factor 1 + 1) * (exponent of factor 2 + 1) * ...
Here's a table showing the prime factorization and number of factors for each perfect square:
Perfect SquarePrime FactorizationNumber of Factors
100 2^2 * 5^29
121 11^23
144 2^4 * 3^225
225 3^2 * 5^29
256 2^89
324 2^2 * 3^415
400 2^4 * 5^225
441 3^2 * 7^29
484 2^2 * 11^29
529 23^23
676 2^2 * 13^29
841 29^23
900 2^2 * 3^2 * 5^227
Counting Numbers with 9 Factors: Finally, we count the number of perfect squares in the table that have exactly 9 factors. There are four such numbers: 100, 256, 441, and 841.
Therefore, there are four 3-digit numbers with exactly 9 factors.