Finding the interest rate

Statement

At what annual simple interest rate does a capital of $1500\ \text{€}$ produce $90\ \text{€}$ of interest in $9$ months?

Solution

From $I = C\cdot r\cdot t$ we get $r = \dfrac{I}{C\cdot t}$, with $t = \dfrac{9}{12} = 0{.}75$ years. $r = \frac{90}{1500\cdot 0{.}75} = \frac{90}{1125} = 0{.}08 = \boxed{8\%}.$