12 : 3 :: 45 : ? Complete the number analogy.

The second term is the biggest prime that divides the first. 12 breaks up as $2\times 2\times 3$, so its largest prime factor is 3; 45 breaks up as $3\times 3\times 5$, so its largest prime factor is 5. The trap 3 is the smallest prime factor of 45, but the smallest for 12 is 2, not 3. 15 is the largest factor of 45 below itself, where 12 would have answered 6, and 9 is its largest square factor, where 12 would have answered 4.