When adding like fractions, why is the numerator the only part of the fraction that changes?

Because youre adding parts of the fraction and not changing the amount of parts the fraction is divided in to.

Well it depends on the problem. If the denominator isn't the same as the first fraction, then their will be a different number. But if its the same, than the numerator is the only part that will change because it adds on unto it..

Ithink not so sure but i tried it out and came out to be b

I'm gonna have to say about 13 percent