Answer:
475
Step-by-step explanation:
The strand is only made up of Adenine, Thymine, Cytosine and Guanine. There will be the same number of Adenine as there are Thymine and the same number of Cytosine as there are Guanine.
We know there are 425 Thymine. This means there are also 425 Adenine. Therefore, you would subtract 850 (425x2) from 1800 to find the amount of Cytosine and Guanine left, which is 950.
Out of the 950, half will be Cytosine and half Guanine. 950/2 is 475.