now, he has 2100 left, he already spent 30% of what he took along, well, 100% - 30% is just 70%, therefore, what he has left, namely the 2100, is really the 70% of the whole amount, let's say he took along "x", thus "x" is the 100%, then,
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
x&100\\
2100&70
\end{array}\implies \cfrac{x}{2100}=\cfrac{100}{70}\implies x=\cfrac{2100\cdot 100}{70}[/tex]
