double ratio(String dna)(be sure your method is public)
dnacontains only the characters 'a', 'g', 't', 'c' (lowercase).
dnais between 0 and 1000 characters long (inclusive).
There is one guanine and one cytosine for two out of five or 40% (which is 0.4)
"catatggatatc"Returns: 0.3333333 (don't worry about round off problems, if your answer is close---within 10-6---it will be correct).