Simple math
C. Find common denominator, and subtract the numerators
However, you need to change the numerator to when you change the denominator.
Example,
2/3 - 1/2
find common denominator of 2 and 3
that will be 6
change numerator by dividing CD to denominator and multiplying numerator by the result.
so numerator of 1/2 will be (6 / 2) * 1 = 3
therefore
1/2 will be 3/6
and
2/3 will be 4/6
then subtract numerator
4 - 3 = 1
therefore
2/3 - 1/2 = 1/6
I hope it helps