If a strand of DNA has 15 cytosines how many thymine will there be?

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The genetic information of a cell is stored in molecules of deoxyribonucleic acid (DNA).
DNA is a polymer of nucleotides . A DNA molecule consists of three parts—a nitrogen base, a five-carbon sugar called deoxyribose, and a phosphate group.

There are four DNA nucleotides, each with one of the four nitrogen bases (adenine, thymine, cytosine, and guanine). The first letter of each of these four bases is often used to symbolize the respective nucleotide (A for adenine nucleotide, for example).

DNA forms a two-stranded spiral, or double helix. Note that adenine always bonds with thymine and cytosine always bonds with guanine.

Your answer is unknown without knowing the number of adenine. Knowing that you have 15 C just means you will have 15 G.

Nothing more.