Question 1111254
Cost  = income is the break even point
Let {{{ n }}} = number of books sold
{{{ 25n + 13200 = 40n }}}
{{{ 15n = 13200 }}}
{{{ n = 880 }}}
--------------
If they sell 880 books they will break even
----------------------------------------
Note that if {{{ n = 879 }}} books, then
cost will  exceed income
{{{ 25n + 13200 > 40n }}}
{{{ 25*879 + 13200 > 40*879 }}}
{{{ 21975 + 13200 > 35160 }}}
{{{ 35175 > 35160 }}}
OK