Question 846746
If profit = {{{ P }}}, then {{{ P = R - C }}}
{{{ P = 13.6x - ( 5.8x + 120 ) }}}
{{{ P = 13.6x - 5.8x - 120 }}}
{{{ P = 7.8x - 120 }}}
Make the profit zero, which means {{{ R = C }}}
{{{ 7.8x - 120 = 0 }}}
{{{ 7.8x = 120 }}}
{{{ x = 15.385 }}}
You can't have a fraction of a frame, 
so the profit is zero when Frank sells
15 frames, and he begins to make a
profit when he sells 16 frames
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check:
{{{ P = 7.8x - 120 }}}
{{{ 0 = 7.8*15.385 - 120 }}}
{{{ 0 = 120.003 - 120 }}}
{{{ 0 = .003 }}}
close enough