Question 396901
Let {{{x}}} = number of notebooks
Let {{{y}}} = number of pens
{{{.75x + .5y = 5}}}
{{{75x + 50y = 500}}}
{{{50y = -75x + 500}}}
{{{y = -(3/2)*x + 10}}} 
I'll plot this
{{{ graph( 400, 400, -5, 13, -5, 13, -(3/2)*x + 10) }}}
The equation shows that only an even number will
work for {{{x}}}. The plot shows this number can only
be 2,4,or 6
The allowed points are (2,7),(4,4), and (6,1)