Question 672460
Let {{{ x }}} = mg of salt to be added
{{{ .2*18 = 3.6 }}} mg of salt already in solution
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{{{ ( x + 3.6 ) / ( 18 + x ) = .35 }}}
{{{ x + 3.6 = .35*( 18 + x ) }}}
{{{ x + 3.6 = 6.3 + .35x }}}
{{{ .65x = 2.7 }}}
{{{ x = 2.7 / .65 }}}
{{{ x = 4.154 }}} 
4.154 mg of salt should be added
check:
{{{ ( 4.154 + 3.6 ) / ( 18 + 4.154 ) = .35 }}}
{{{ 7.754 / 22.154 = .35 }}}
{{{ 7.754 = .35*22.154 }}}
{{{ 7.75 = 7.7539 }}}
close enough