In sea urchin DNA, which is double stranded, 17% of the bases were shown to be cytosine. The percentages of the other three bases expected to be  present in  this DNA are 

In a double-stranded DNA, the amount of adenine is equal to thymine and the amount of guanine is equal to cytosine (Chargaff’srule).
If cytosine is 17%, guanine is also 17%. C + G together account for 17 + 17 = 34% of DNA.
In the remaining 66%, adenine and guanine are present in equal proportions, i.e., 33% and 33%.
Hence, the amount of guanine is 17%, adenine is 33% and thymine is 33%.