$A$ does the job in $10$ seconds.
$B$ does the job in $5$ seconds.
$C$ does the job in $7$ seconds.
Is $A$ $100\%$ worse than $B$?
Is $B$ is $100\% $better than $A$?
What percentage is $C$ better than $A$?
Edit: For a little bit of background; I'm trying to compare the running times of two algorithms.
Yes, B is twice as good as A. Equivalently, since: $$ \left(\frac{10}{5} - 1\right) \cdot 100~\% = 100~\% $$ we know that B is $100~\%$ better than A. Likewise, since: $$ \left(\frac{10}{7} - 1\right) \cdot 100~\% \approx 42.86~\% $$ we know that C is about $42.86~\%$ better than A. Equivalently, C is about $1.4286$ times as good as A.