well, let's say it was "x", so that's the 100% then. 312 is today's price, which is 20% more than last year's, therefore 312 is really 100% + 20% or 120%.
if 312 is the 120%, what is "x" anyway?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
312&120\\
x&100
\end{array}\implies \cfrac{312}{x}=\cfrac{120}{100}\implies \cfrac{312\cdot 100}{12}=x[/tex]
