well, the increase from 1200 to 1600 is just 400.
if we take the 1200 to be the 100%, what is 400 off of it in percentage?
[tex]\begin{array}{ccll} amount&\%\\ \cline{1-2} 1200&100\\ 400&x \end{array}\implies \cfrac{1200}{400}=\cfrac{100}{x}\implies 3=\cfrac{100}{x}\implies 3x=100 \\\\\\ x=\cfrac{100}{3}\implies x=33.\overline{3}[/tex]
