Base pairing in DNA follows Chargaff’s rule. Cytosine on on strand pairs with guanine on other strand by triple bond. So, if you have 40 cytosine in a sequence of DNA, then number of guanine will be required is 40.
If the sequences of the bases in one strand of the DNA is AGCTATA, then the the sequence of the bases on the other strand will be complimentary to the bases on first strand. Bases sequence will be TCGATAT.