Question 943900
( water ) / ( fertilizer ) = {{{ 5/2 }}}
( water + fertilizer ) = 5 litres
( fertilizer ) = ( 5 - water ) litres
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Let {{{ w }}} = litres of water needed
{{{ w / ( 5 - w ) = 5/2 }}}
{{{ w = ( 5/2 )*( 5 - w ) }}}
{{{ 2w = 5*( 5 - w ) }}}
{{{ 2w = 25 - 5w }}}
{{{ 7w = 25 }}}
{{{ w = 3.571 }}} 
3.571 litres of water are needed
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check:
{{{ w / ( 5 - w ) = 5/2 }}}
{{{ 3.571 / ( 5 - 3.571 ) = 5/2 }}}
{{{ 3.571 / 1.429 = 5/2 }}}
{{{ 2*3.571 = 5*1.429 }}}
{{{ 7.142 = 7.145 }}}
OK
error due to rounding off, I think
also:
( fertilizer + water )  = 5 liters
{{{ ( 5 - w ) + w  = 5 }}}
{{{ 1.429 + 3.571 = 5 }}}
{{{ 5 = 5 }}}
OK