Capital from the simple interest

Statement

A capital produces $126\ \text{€}$ of simple interest in $7$ months at $9\%$ per year. What is the capital?

Solution

From $I = C\cdot r\cdot t$ we get $C = \dfrac{I}{r\,t}$, with $t = \dfrac{7}{12}$. $C = \frac{126}{0{.}09\cdot \frac{7}{12}} = \frac{126}{0{.}0525} = \boxed{2400\ \text{€}}.$