Question 900304
Let {{{ n }}} = the number of brownies in the tray
at the start
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The 1st customer takes 
{{{ n/2 + 1/2 }}} brownies
That means there are 
{{{ n - ( n/2 + 1/2 ) }}}
{{{ n/2 - 1/2 }}}
brownies left
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The 2nd customer takes 
{{{ ( 1/2)*( n/2 - 1/2 ) + 1/2 }}}
{{{ n/4 - 1/4 + 1/2 }}}
{{{ n/4 + 1/4 }}}
That means there are
{{{ ( n/2 - 1/2 ) - ( n/4 + 1/4 ) }}} 
{{{ n/2 - 1/2 - n/4 - 1/4 }}}
{{{ n/4 - 3/4 }}}
brownies left
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The 3rd customer takes
{{{ (1/2)*( n/4 - 3/4 ) + 1/2 }}}
{{{ n/8 - 3/8 + 4/8 }}}
{{{ n/8 + 1/8 }}}
That means there are
{{{ ( n/4 - 3/4 ) - ( n/8 + 1/8 ) }}}
{{{ n/4 - 6/8 - n/8 - 1/8 }}}
{{{ n/8 - 7/8 }}}
brownies left
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You are told also that
{{{ n/8 - 7/8 = 0 }}}
{{{ n/8 = 7/8 }}}
{{{ n = 7 }}}
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There were 7 brownies before 1st customer came in
check:
1st customer took 
{{{ 7/2 + 1/2 = 4 }}}
There were
{{{ 7 - 4 = 3 }}} left
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The 2nd customer took
{{{ 3/2 + 1/2 = 2 }}}
There were 
{{{ 3 - 2 = 1 }}} left
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The 3rd customer took
{{{ 1/2 + 1/2 = 1 }}}
There were
{{{ 1 - 1 = 0 }}} left
OK