Let 
be the number of liters of the 10% solution to be used, and 
the number of liters of the 4% solution. The chemist wants to end up with a 4 liter solution, so
For each liter used in the mixture, a concentration of either 10% or 4% acid will be contributed, and the goal is to make a 4L solution whose concentration is 7%, which means
Solving for 
gives 
, so 2L of both solutions is needed.
Temperature at mid-night
=235+(-485)
=-250 degrees, whatever the unit was (C,F, R, K).
718,956;716,480;715,230;714,398;713,486;712,396;710,469;709,612;708,421;706,329
