mixed numbers subtraction vertically In the following subtraction we are subtracting $2$ mixed numbers vertically. I know how it works except the last step.
$$ 7 \frac{1}{3} - 4 \frac{1}{2} = 3 + \frac{-1}{6} = 2 + \frac{5}{6} = 2 \frac{5}{6}$$
I am confused about this step: $ 3 + \frac{-1}{6} = 2 + \frac{5}{6}$
How does this work? 

Write it as
$$ 3 - \frac{1}{6}= 2 + 1 - \frac{1}{6}= 2 + \frac{6}{6} -\frac{1}{6} = 2 + \frac{6-1}{6} = 2+\frac{5}{6} $$
