If a DNA molecule is found to be composed of 40% thymine, what percentage of guanine would be expected.

10%

Explanation:

Chargaff postulated the complementary base pairing rule, where he stated that in the DNA molecule, Adenine (A) will always pair with Thymine (T) and Guanine (G) with Cytosine (C) i. e. A-T, G-C

Chargaff stated that the concentration of purines (Adenine and Guanine) is always equal to that of pyrimidines (Thymine and Cytosine) i. e. {A} + {G} = {C} + {T}

These equal concentration equals 100% in totality i. e. A+G+C+T = 100%

Thymine pairs with Adenine, if there is 40% of Thymine in the DNA molecule, it means there will be 40% of Adenine also. 40 + 40 = 80%. This leaves us with 100-80 = 20% for Guanine and Cytosine.

Since there will be an equal amount of Guanine and Cytosine too, Guanine will therefore, be 20/2 = 10%.

That is, 10% Guanine, 10% Cytosine.