Question

If you have a strand of DNA in which 20% of the bases are adenine, what percentage of guanine do you have?

Answer 1

The perecentage of guanine is 30%

Explanation:

According to the Chargaff's rules the DNA from any cell of all organisms should have a 1:1 ratio (base Pair Rule) of purine bases [for the DNA cytosine, thymine and for the RNA uracil] and pyrimidine bases [guanine and adenine for RNA and DNA]. The amount of guanine should be equaled to cytosine and the amount of adenine should be equaled to thymine. You can follow this rule in both strands of the DNA.

As you can see above [look at the colors and the bases of the DNA] the purines always pair with the pyrimidines. Cytosine pairs with guanine and adenine pairs with thymine.
Following this you can solve your DNA percantage problem. I suggest you to draw the bases of the DNA chain to visualise the task. Here below you can see how I didi it.

Let`s get to the point!
You have 20% of adenine. If you have 20% of adenine than you have 20% of thymine, because the amount of adenine and thymine is equal. 20% plus 20% is 40% of adennine and thymine. From 100% DNA bases subtract 40% and you will get 60%. Then divide this by 2 and you will get 30%. 30% of guanine and 30% of cytosine, because their amounts are equal in the DNA stand.