well, it went from 400 to 550, so the difference is 150.
if we take 400 to be the 100%, what is 150 off of it in percentage?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
400&100\\
150&p
\end{array}\implies \cfrac{400}{150}=\cfrac{100}{p}\implies p=\cfrac{150\cdot 100}{400}[/tex]
