Question

If a DNA molecule is found to be made of 40% thymine,What percentage guanine would you expect

Answer 1

2 types of base pairs in DNA: 
G-C 
A-T 
Since 40% is T, an equal amount will be A (40%+40%=80%)
20% is left, and this must also be equal, so G is 10% (C is the other 10%)