Question 408969
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Hi
Let x represent the amount drained off and replaced
Question states***
  .20(16qt-x) + 1*x = .25*16qt
Solving for x
   .8x = .05*16qt
     x = 1qt, needs to be drained off and replaced with pure antifreeze

CHECKING our Answer***
  .20*15 + 1 =  4 = .25*16