What goes into 15 that goes into 54?

The numbers that go into both 15 and 54 are their common divisors. The divisors of 15 are 1, 3, 5, and 15, while the divisors of 54 are 1, 2, 3, 6, 9, 18, and 27. The only common divisor between the two is 1 and 3. Therefore, 3 is the answer, as it is the highest common factor that goes into both numbers.