Here's why:

* Base Pairing Rules: In DNA, adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This is known as complementary base pairing.

* Equal Proportions: Since each base has a specific partner, the number of adenine bases in a DNA molecule will always be equal to the number of thymine bases. Similarly, the number of guanine bases will always be equal to the number of cytosine bases.

* Percentage Calculation: Because the number of each base pair is equal, their percentages within the DNA molecule will also be equal.

Let me know if you'd like more explanation on any of these points!