Question 709788
If you understand a formula, you can 
set up a table
Let {{{ w }}} = ml of pure water to be added
{{{ .04*100 = 4 }}} ml of salt in 4% solution
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{{{ 4 / ( w + 100 ) = .025 }}}
{{{ 4 = .025*( w + 100 ) }}}
{{{ 4 = .025w + 2.5 }}}
{{{ .025w = 1.5 }}}
{{{ w = 60 }}}
60 ml of pure water must be added.
Note that there is no change in the amount of salt
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check answer:
{{{ 4 / ( 60 + 100 ) = .025 }}}
{{{ 4 / 160 = .025 }}}
{{{ .025= .025 }}}
OK