all the thymine pairs with adenine, therefore there is the same number of adenine and thymine bases and the same percentage
so we have 28%thymine and 28% adenine
100-28-28=44% left
this 44% is guanine and cytosine. All of the guanine pairs with cytosine and all of the cytosine pairs with guanine so we have 44/2=22% cytosine and guanine
composition:
28%adenine
28%thymine
22%guanine
22%cytosine