Who discovered that the amount of adenine in DNA always equals the amount of thymine?
Those are Chargraff's Rules.
You're wrong to think that adenine and thymine are the only
pairs however because in RNA, adenine always pairs with uracil.
Thymine is not in RNA.
Adenine and Guanine are purines. Cytosine, thymine, and uracil
are purimidines.
Sources: My Science class and textbook
Hope this helped!