Question 727984
After 1st replacement,
there is {{{ 20 - 5 = 15 }}} liters of acid in 20 liters of solution
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After 2nd replacement
There is {{{ 5*( 15/20 ) = 15/4 }}} liters of acid removed
There is {{{ 15 - 15/4 = 45/4 }}} liters of acid left
{{{ 11.25 }}} liters left
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After 3rd replacement
There is {{{ 5*( ( 45/4)/20 )  = 5*(45/80) }}}
{{{ 45/16 }}}  liters of acid removed
There is {{{ 45/4 - 45/16 = 135/16 }}} liters of acid left
{{{ 8.4375 }}} liters left
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After 4th replacement
There is {{{ 5*( ( 135/16 ) /20 ) = 135/64 }}} liters 
of acid removed
There is {{{ 135/16 - 135/64 = 405/64 }}} liters of acid left
{{{ 6.328125 }}} liters left
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After 5th replacement
There is {{{ 5*( ( 405/64 ) /20 ) = 405/256 }}} liters of acid removed
There is {{{ 405/64 - 405/256 = 1215/256 }}} liters of acid left
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{{{ 1215/256 = 4.7461 }}} liters of acid left
Hope I got it!