13 / 6 = 2.16666666 (6's forever) = about 2.167
2.167 in % is 216.7%
You took 216.7% as many classes as you wanted.
You wanted to take 100% of the classes you wanted.
If you took 216.7% of the classes you wanted instead of 100% of the classes you wanted, then you took 216.7% - 100% = 116.7% too many.