Question 839179
The 3 rings weigh {{{ 3*18 = 54 }}} g
The weight of the silver is {{{ .07*54 = 3.78 }}} g
The weight of the gold is {{{ .93*54 = 50.22 }}} g
Let {{{ x }}} = grams of silver to be added 
to reduce the gold content to 81%.
----------------------------------------
If the gold content is to be 81%, the silver content
will be {{{ 100 - 81 = 19 }}}%
{{{ ( x + 3.78 ) / ( x + 54 ) = .19 }}}
{{{ x + 3.78 = .19*( x + 54 ) }}}
{{{ x + 3.78 = .19x + 10.26 }}}
{{{ .81x = 10.26 - 3.78 }}}
{{{ .81x = 6.48 }}}
{{{ x = 8 }}}
-----------------
8 g of silver must be added
check:
{{{ ( x + 3.78 ) / ( x + 54 ) = .19 }}}
{{{ ( 8 + 3.78 ) / ( 8 + 54 ) = .19 }}}
{{{ 11.78 / 62 = .19 }}}
{{{ 11.78 = .19*62 }}}
{{{ 11.78 = 11.78 }}}
OK