Question 760463
Let {{{ x }}} = the selling price of 1st watch
Let {{{ y }}} = the selling price of the 2nd watch
I'll say he makes a profit, {{{ P }}}
{{{ P = .2x - .2y }}}
{{{ x + y = 240 }}}
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{{{ y = 240 - x }}}
{{{ P = .2x - .2*( 240 - x ) }}}
{{{ P = .2x - 48 + .2x }}}
{{{ P = .4x - 48 }}}
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If the profit is zero, the {{{ x = 120 }}} 
If {{{ x = 240 }}}  ( {{{ y = 0 }}} )
{{{ P = 96 - 48 }}}
{{{ P = 48 }}}, which is 20% of 240