This questions requires knowledge of the base pairing in DNA, plus a little bit of maths! As you know, bases on the 2 DNA strands always pair in the same way: A (adenine) with T (thymine) , C (cytosine) with G (guanine). I will use an example to explain.

Say you are told that strand 1 is 30% A, 25% C and 35% T. A always pairs with T, and so there will be the same amount of T on strand 2 as there A on strand 1 i.e. 30%.

25% C on strand 1 means there is 25% G on strand 2, as these bases always pair together. Strand 1 is 35% T and so strand 2 must be 35% A.

Finally, on strand 1, the total percentages must equal 100 and so this leaves 10% to be G. 10% G on strand 1 means 10% C on strand 2.