Question 1055896
The cost of the orignal {{{ 20 }}} liters of milk
was {{{ 3.5*20 = 70 }}} 
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After adding water ( I am assuming water costs nothing )
there are {{{ 20 + 4 = 24 }}} liters of mixture
{{{ 70 / 24 = 2.9167 }}}
rs 2.92 average cost/liter