Assume that a 100 base pair dna double helix contains 4 cytosines how many adenine and guanine are there

G pairs with C
A pairs with T

45 cytosines, 45 guanines
55 adenines, 55 thymines 
Cytosine pairs with Guanine, and Adenine pairs with Thymine.
This means, that if there are 45 cytosines, there must be 45 guanines.
So that's 90 base pairs down.
That means that there are 10 left between adenine and thymine- so that means there are 5 adenines.