Question 1135518
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Here is a shortcut alternative to the traditional algebraic solution method shown by the other tutor.  If you understand it, it will get you to the answer to any mixture problem like this much faster and with far less effort.<br>
(1) You are starting with 20% antifreeze and adding 40% antifreeze, until the mixture is 35% antifreeze.
(2) 35% is three-fourths of the way from 20% to 40%; therefore, 3/4 of the mixture needs to be what you are adding.
(3) So 1/4 of the mixture is what you started with; since that was 100 gallons, you need to add 300 gallons of the 40% antifreeze to get the desired mixture.