Question 698523
Let {{{ x }}} = liters of water to be added
{{{ .85*20 = 17 }}} liters of ammonia in 85% solution
The key is that you have the same liters of ammonia
at the end as you started with.
{{{ 17 / ( 20 + x ) = .7 }}}
{{{ 17 = .7*( 20 + x ) }}}
{{{ 17 = 14 + .7x }}}
{{{ .7x = 3 }}}
{{{ x = 4.286 }}} 
4.286 liters of water need to be added
check:
{{{ 17 / ( 20 + 4.286 ) = .7 }}}
{{{ 17 / 24.286 = .7 }}}
{{{ 17 = .7*24.286 }}}
{{{ 17 = 17 }}}
OK