You have to multiply the two fractions by each other:
\(\frac{3}{4} * \frac{2}{5}\)
you mupliply the nominator and denominator separately:
\(\frac{3*2}{4*5}\)
which is
\(\frac{3}{2*5}\)
which is finally
\(\frac{3}{10}\)
so you ate
\(\frac{3}{10}\) or 0.3 of the box!