$\frac{1}{3}x + \frac{1}{5}x + \frac{1}{6}x = \frac{21}{40}$
$\frac{10 + 6 + 5}{30}x = \frac{21}{40}$
$\frac{21}{30}x = \frac{21}{40}$
$x = \frac{21}{40} : \frac{21}{30}$
$x = \frac{21}{40} * \frac{30}{21}$
$x = \frac{1}{4} * \frac{3}{1}$
$x = \frac{3}{4}$

$\frac{1}{4}x + \frac{1}{6}x + \frac{1}{8}x = \frac{39}{56}$
$\frac{6 + 4 + 3}{24}x = \frac{39}{56}$
$\frac{13}{24}x = \frac{39}{56}$
$x = \frac{39}{56} : \frac{13}{24}$
$x = \frac{39}{56} * \frac{24}{13}$
$x = \frac{3}{7} * \frac{3}{1}$
$x = \frac{9}{7} = 1\frac{2}{7}$