Answer:
x is mL of 25% acid and y is mL of 50% acid
We set up 2 equations:
A) x + y = 250
B) .25 x + .50y = .35 * 250 which equals
B) .25x + .50y = 87.50  Multiplying A) by -.5 
A) -.5x -.5y = -125  adding the equations we get
-.25x = -37.50
x = 150 mL
y = 100 mL
Source:  http://www.1728.org/mixture.htm
Step-by-step explanation:
 
        
             
        
        
        
1 kg--> 0.264 kg fat
:. 12.5 kg--> (0.264*12.5) 3.3 kg fat
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Constant rate means its the same rate and didn't change over time
        
             
        
        
        
It is a whole number, and is positive