Why does frankenstein feel he has the right to take the life of his monster ?

Because he is the one who created him and he feel responsible for him or like he owns him.

Fraction division is basically multiplication of a reciprocal.

For example (1/4)/(1/8) = (1/4)x(8/1)=2

Multiplication of the answer and the piece that divides (in this case 2 x 1/8), you should get the piece that is being divided (in this case 1/4). If that is correct, then your division is correct.