How many guanines are found in a ds DNA that has 700 hydrogen bonds and 10% adenines?

Dear Student!

If the percentage of Adenine is 10% then the % of Thymine would also be 10% as they base pair by two hydrogen bonds with each other. Guanine makes 3 hydrogen bonds with cytosine and they make up 90% of the DNA.

Lets make some equations: 

Assume number of A=T to be x and G≡C to be y, then according to the problem, 

2x + 3y = 700; because A=T makes 2 hydrogen bonds and G≡C makes 3 hydrogen bonds

we can make another equation

y = 9x; because the DNA has 10% A=T  and 90% G≡C

By solving these two equations the number of guanine residues come out to be approximately 217

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