An algebra problem by Tanirika Majumder

$let$ $x = \text{total money},$ $m = magnet$ $and$ $p = \text{post card}$

$\frac{1}{3}x = 5m + 11p$ $\color{#20A900}(1)$

$m = 3p$ $\color{#20A900}(2)$

Substitute $\color{#20A900}(2)$ in $\color{#20A900}(1)$ $\implies$ $\boxed{x = 26m}$

Then she bought some magnets with $\frac{3}{4}$ of the remaining money. The remaining money is $x - \frac{1}{3}x = \frac{2}{3}x$ . Then, $\frac{3}{4}$ of the remaining money is $\frac{3}{4}(\frac{2}{3}x) = 0.5x$ .

Then we have

$(0.5x)(\frac{26m}{x}) = 13m$

$\text{total magnets}=$ $13m + 5m=$ $\large\color{#D61F06} \boxed{18m}$