he already practiced 40%, he needs to do the rest, the remaining 60%.
now, we know that 40% is 12 minutes, what is the 60% anyway?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
12&40\\
x&60
\end{array}\implies \cfrac{12}{x}=\cfrac{40}{60}\implies \cfrac{12\cdot 60}{40}=x[/tex]
he needs to practice an extra "x" minutes.
