If there is 42% adenine in one DNA chain, how much guanine will there be in the antiparallel chain?

By the principle of complementarity:
The nitrogenous base – Adenine (A) will correspond to the nitrogenous base – Thimin (T). This means that their number and percentage will be equal. A = T, 42 = 42.
Since there can be only 100 percent of nitrogenous bases in the chain, we subtract 42 + 42 = 84 from 100
100-84 = 16 percent Guanine (G) and Cytosine (C) of this chain.
According to the principle of complementarity, G = Ts. So 16/2 = 8.
The antiparallel chain will coincide in percentage with the given one initially, for this reason the content of G = 8.