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Echinoderms, like chordates, are deuterostomes and are therefore thought to be the most closely related of the major phyla to the chordates
In DNA, thymine binds to adenine, and cytosine binds to guanine. This means that there is an equal amount of thymine and adenine, and there is an equal amount of cytosine and guanine.
If there is 15% thymine, there should be 15% adenine.
Note that in real life, the percentage of bases won't be 100% equal.