Question 640219
Let {{{ x }}} = ounces of pure acid needed
A gallon is {{{128}}} US fluid ounces
given:
{{{ .05*128 = 6.4 }}} ounces of acid in gallon of vinegar
{{{ ( 6.4 + x ) / ( 128 + x ) = .06 }}}
{{{ 6.4 + x = .06*( 128 + x ) }}}
{{{ 6.4 + x = .96 + .06x }}}
{{{ x - .06x = 7.68 - 6.4 }}}
{{{ .94x = 1.28 }}}
{{{ x = 1.362 }}} 
1.362 ounces of pure acid are needed
check:
{{{ ( 6.4 + x ) / ( 128 + x ) = .06 }}}
{{{ ( 6.4 + 1.362 ) / ( 128 + 1.362 ) = .06 }}}
{{{ 7.762 / 129.362 = .06 }}}
{{{ 7.762 = .06*129.362 }}}
{{{ 7.762 = 7.762 }}}
OK
If you don't use US fluid ounces, just change
that gallons to ounces conversion